When should you give up top tower for Dragon?

Here's a rather deep question for those of you who play a lot of ranked. Without teleport, I have no real way of reaching my team short of physically going there from top lane. My question is this:

I have no real way of pushing my opponent out of lane or pushing hard enough so as to pressure his tower. When should I simply leave lane and help my team get Dragon at the risk of losing my top tower, assuming that my presence is needed?

Clarification Edit: I'm talking about the tier 1 tower during the laning phase.

Purely speaking of dragon vs tower. The dragon is a better option.

• The dragon gives more gold. (750 vs 975).
• What you lose from the tower will be won back once you killed the enemy tower.
• The enemy team wont be killing the dragon over the next 6 minutes, you will not have to defend that dragon.
• You don't need to defend that tower either.

That said, dragon is not the top lane responsibility.

It's strongly situational. If you can save both, just save the tower first and then do dragon. If the enemy or minions are killing bot or mid tower, it's easy to defend it. You probably already have 3 people at the dragon, and now you simply gank bot or mid before you go back to dragon. Or dispatch 1 and do the dragon with just 2 people.

If we're speaking of the first 3 towers, those will fall eventually anyway, and are not worth that much post laning phase. If we're talking inhibitor towers or base tower, you MUST defend those before doing dragon.

At the very least, make sure you keep the dragon warded and keep the enemy team away from it. That is a good job for the jungler and the support. If i had to choose between killing a enemy tower, or preventing my enemy from killing the dragon, i would certainly save the dragon.

Getting dragon and keeping tower is of course the best option. The responsibility for killing the dragon lies at the bot laners early game. They should push or win there lane, and then kill dragon. If any jungler is around, the route around level 6 or 7 should be either gank bot->dragon or just call bot->dragon. If it's not possible to do the dragon at this point because of enemy aggression (and map control), keep the dragon warded and prevent the enemy team from taking it instead.

Meta has changed so I figured I should add something slightly different in here.

First dragon is always worth losing your t1 tower for. The first dragon is a very nice strong buff for your entire team. After this you have different scenarios coming up.

1) You have a dragon but the enemy team does not:

This is the hardest scenario for me to figure out. On one hand, you want to stop the enemy from getting that buff, on the hand you can deny the tower and potentially put pressure on top. If it's the first tower, I might lean on giving it up just because as the game goes on that bonus feels better and better with items. If you can deny them every dragon it's pretty huge.

2) The enemy team has a dragon but you do not:

This is a bit risky. Only do this if your team is ahead otherwise or you know enemy laners won't be able to outnumber you. If it's a completely even fight, they will have the better chance of winning since they have that first dragon buff and your team doesn't.

3) You both have a dragon:

If you both have a dragon, you are better off going for the tower. Consecutive dragon's are worth that much unless it's the 5th one. By that point though I expect everyone's t1 towers to be down unless it's a 1 sided game.