Well, seeing as I'll be 39 when I START instructing, let me interject my opinion:
NEVER turn down an opportunity (and almost everything that DE727UPS said)!
When I set out on this endeavor, the airlines were THE goal for me. However, after being around several pilots and looking into other avenues of aviation, I'm also GREATLY interested in the coporate side of things, so when I start meeting Part 135 mins, I'll start applying to:
The Regionals
Charter
Small freight (like AirNet)
Flying dog poop out of North Dakota
ANYTHING to get me into a paid multi-engine and/or turbine time (read: Caravan) flying position.
Then, I'll see where that leads me. My goal is to be sitting right seat in some sort of jet by the age of 45.
At any rate - to answer your question - yes, if a regional airline with whom I interviewed offered me a slot - but was not my "dream regional", I'd accept.
Just one man's opinion.
R2F