Dash's ability states that "you may ignore obstacles" which implies that you may choose not to ignore obstacles.

That said, if I do a boost that ends up on an asteroid could I say that I'm not ignoring it and reverse the action?

If I then barrel roll through an asteroid instead (template overlaps) could I ignore the second obstacle after not ignoring the first one?

First question, yes - it is not mandatory so if it is better to not ignore them, you may.

Second question - interesting question - is it a one time trigger and you must choose to ignore all obstacles or none? Hmm. My gut says your scenario would work, you could choose to not ignore the asteroids when boosting, and then decide to ignore them when barrel rolling. I don't really have anything in the rules to back that up but it seems like it should work that way.

This is covered in the FAQ.

Dash Rendar

When declaring a barrel roll or boost, Dash Rendar must choose whether or not to use his ability before measuring if he would overlap an obstacle.

So I'm guessing he can declare separately for each barrel roll/boost since he didn't have to declare it before the maneuver.

So you have Dash with Engine Upgrade, and Push the Limit.

You take as your standard action barrel roll and then as your free action from PtL boost. I'd say yes you'd have the option of declaring your intent for either case, so you could say yes for the barrel roll but no for the boost.

I seem to recall something saying you can choose to ignore the ability. In the case of a boost/roll you say you're not using it and then check to see if you have a legal move; if you don't then you get to back up and can declare something else and apparently use the ability on a different action.

Personally, I believe it should either be on or off during any given phase. Although it may be legal I have a hard time with allowing it to be used when making a maneuver that carries you over, and perhaps onto, an asteroid and then saying you're NOT going to use it as you check for boosts/rolls that could get you off the rock but allow you to avoid "accidentally" landing on another one.