Most useful destruction energy type and managing perks

I'm playing a pure mage build (might try to do the thief/assassin quests when done with the mage quests).

Right now, I use perks sparingly. There are perks specific for fire, electricity, cold based destruction spells.

I'm wondering if one energy type fits all in this game. Should I just focus on electricity? Or do certain energy types work better against certain opponents? I think I read that dragons are vulnerable to cold, but ice-trolls are vulnerable to fire, so my suspicion is that I'm going to need to invest in all of them.

At the same time, is it wise to be careful with one's perks? I summon a lot of flame antorachs, but plan on upgrading the monster I summon to the next level (whatever comes after apprentice). So should i skip the perk that lessens the cost of apprentice level summons and just get the next one? Is that even possible? I haven't tried to skip a perk in a chain in the various asterisms...

My mage knowledge is mostly second hand but I can give out these pointers so far:

• You can not get a higher tier perk with out getting the lower ones. For example you can do nothing in the destruction tree with out starting with the Novice perk. This applies for all of them as well.
• There is no end all be all elemental skill. A lot of people find Electrical the most useful simply cause it instantly strikes what is under your target, you do not have to lead your target. But Ice works against fire based dragons (and I've found it very useful against giants), fire works against both kinds of trolls, all of the undead, and electrical works good against mages. If you are trying to be an offensive mage you will really want to go with all of the elements in the long run so you are never stuck with just one choice.
• Impact is pretty much a Must Have. When my brother got this he went from having to do some fancy kiting of enemies to being able to constantly stumble them. This is -not- the win button, but it definitely puts you on a bit more even footing than you are with out it.
• When considering Conjuration keep in mind that the max level summons, known as thralls, are permanent until they die. You will need to have enough mana on hand to cast them, but it may not be worth while to put perks in here aside from those that allow you access to have 2 summons at once.

I hope this high level overview has been helpful and addressed your questions. Any more concerns ask in the comments and I will try and add in more information as I currently understand it.

If you are going pure mage: Destro, be sure to spread out your perks... Do not focus on one specific type of cast (IE just fire). There are several creatures in the skyrim world that have immunities and resistances to each of those: Frost, Fire, and electric.

Be sure to get the stun perk and a dual casting perk in which ever specialization you see fit. My Recommendation: Fire, as it does great damage (Ranking from lowest to highest amount of damage done: Frost, Fire, electric), doesn't waste all of your magika instantly, and stuns your target for easier kiting when combined L&R hand. (REMEMBER: you must have flame or your dual specced casting ability in both your left and right hand, then cast at the same time for even more damage via those perks).

Even though you might not put points in conjuration: if you are having a tough fight, call on a atronach or your partner to tank or pull 'aggro' off of you, while you nuke your opponent.

As a mage, you will have to use your magika wisely or else you will be kiting enemies for hours. My recommendation is to boost your magika to around 300-310 and then focus your gear on magika regen. A big pool of magika will be pointless if you cannot rebuild it fast enough.

Hope this helps.

EDIT:

You cannot skip items in your available perks path. You must unlock the pervious one to it, for the next one to be available. But in circular perk paths - such as blacksmithing - you can choose to go one way as appose to the other (left / right).

You can always save your perks for later, but be sure to grab the health / stam / magika bonus as soon as possible

It's probably a bug, but the Illusion perk "Aspect of Terror" makes fire spells do +10 points of damage. So for example, my character with Aspect of Terror and the first level of Augmented Flames (1/2) does 41 points of damage with his Firebolt spell and 20 (!) points of damage with the formerly weenie Flames spell. That's right, 20 points of damage per second from Flames.

I guess Fire spells have a Terror quality to them whether you have the Searing Flames perk or not, which is why Aspect of Terror affects them, but I'm sure this is a bug. A very fun bug though =) this is still current in version 1.5.

Since everyone seems to be suggested you spread those perks around, I need to be the devil's advocate here. There are two good reasons to pick one: you'll probably be using that element 90% of the time anyway, and it won't matter late-game since the better builds will either have Destruction as support for summons or abandoned entirely.

While it's true that not one element fits all, that's not necessarily true for perks. I can speak from my experience using fire that while some enemies have elemental resistances that doesn't mean you should throw perks into everything on the Destruction tree. This is especially because Destruction damage does not scale; a level 30 mage casting Thunderbolt is doing the same amount of damage as a level 55 mage (it just costs a bit more for the lower level mage). Damage output becomes an issue in late-game and harder difficulties. You eventually get into fights like this. You're more likely to use Destruction (if at all) as support for your follower and summons by stun-locking enemies, in which case elemental choice doesn't matter much.

If I were to recommend one, it would depend on whether you want to hit undead and trolls hard and cast cheaply (fire) or want something easier to aim but requiring more magicka (shock). Ice slows enemies, but I don't like how many are resistant to it, including the natives. I would recommend focusing on one perk-wise but still having all spells available.

You may find that by the end of the game you like being a pure attack caster, but aside from those rare souls it's better to be conservative with Destruction perks as its usage can change heavily.