You really need to go by the information about the plane that is given to you. Open up its information panel and look at its anti-air strength and defence.
All planes have anti-air, and that helps with Air Superiority. If you send low/high level bombers which are very low, it will add to AS, but not much. Against just a few fighters with high anti-air and enemy AS is going to be superior.
Each type is rated anti-personel, anti-tank, anti-ship, etc. You should use their strengths in the obvious role. Interdiction is good for long range, though care should be taken with fighter cover range. CS is good for shorter range and those with high attack values, and of course, keep fighters on AS and in range to cover it all. Some planes are particularly good at bombarding naval craft, remember those and position accordly.
It is always important at the start of the scenario to review air assets and consider what is best for what mission, and of course, your goals and the present condition of the enemy.
It is totally up to you and if you want level bombers on AS, go ahead. Won't help that much, and its waste of their strengths, but your in command.
I would personally not turn on the Air Assistant if you don't have to. It sure has a tendency to attack air bases and other targets with high AA and take some really heavy losses. I would prefer for a future air assistant that only setup modes, rest, interdict, AS, etc. (does a pretty decent job), but NEVER performs a bombardment mission unless I set it manually. I would use it then, maybe in a future update. For now, I keep it off and preserve my air strength, or sacrifice it myself on something of significance.