Should you go on every crash recovery mission?
The game can become quite grindy as it goes on, for crash recovery missions.
There's the risk that you'll start taking risks, when bored of the mission, and lose men.
Should you go on every crash recovery mission, or start using air strikes?
Solution 1:
Recovery missions are riskier than an airstrike. While they do provide roughly twice the income, they also carry a risk of taking casualties. Also from experience: the difference in income just about covers the price of hiring new soldiers to replace dead ones. On the other hand, surviving soldiers receive experience and attribute increases. In the end, it is up to you to decide whether that risk is worth taking, and that depends on your playstyle, difficulty and game mode.
In normal mode, it boils down to your patience and/or willingness to save scum your way to victory. In ironman, you need to adjust the recoveries you attempt according to your losses. Unless there's new technology to be acquired from the mission, it's relatively safe to bomb the site.
Solution 2:
Early game
Yes, each and every one. You'll get some deaths and injuries, but you should design your line up to be able to constantly do them. Hire more people, build a medical center, and so on. For the early game, it's often even worth travelling up to 24 hours away to a crash landing on the other side of the planet at night, and doing a second one.
Early game cash stacks on a lot. You'll want more bases, radars, and interceptors as soon as possible. And you want the first armor as fast as possible, to the point it's worth sacrificing a few rookies to grind for it.
More importantly, early crashes are the best form of training. Rookies are very fragile. You'll want them to get enough HPs/reaction to not get one-shotted. You'll need enough TUs, accuracy, and strength for them to be useful later on. They don't have to be top tier, but they should each at least get a few kills under their belt.
It's also good training for you, as the commander. If you're rushing and making mistakes, find a way to be more cautious. You'll also find a way to sweep the map faster and safer while keeping civilians alive. This is a vital skill for later missions, especially the deadly ones where you're left with only a couple of people at the end of the round.
Later game
Once your soldiers have about 5 kills each, the risk and reward both drop drastically. At this point you can decide not to do them if you so wish.
The medium sized and above ships are far riskier, especially the ones where the aliens fortify themselves in the ship and require you to storm it. You might consider skipping those.
Speed tips
My early game squad build is to have a small assault team to kick in doors and search for aliens, while another support team holds a good location, like the center of a map or building with height. Support team holds heavier firepower, like sniper, MG, rocket. I can safely scout the map controlling only 4 people most of the time. Some enemies can be fished or funneled into the MG/sniper nest.
Also keep in mind that 80% of your goal is to just capture the ship. If you have someone in the ship for several rounds, you win the mission. You don't have to hunt down every last alien.