Flight instructing counts as commercial flying and therefore to your quarterly and yearly totals.
It's kind of sour for your primary employer to not be able to schedule you because of the amount of flying you do on the side.
Also consider accidents, incidents or violations affecting your main job and your airline career.
I've chosen not to for several reasons;
I'd consider teaching ground school as that's something I've always enjoyed anyway but no flying.
Not too interested in SE flying for now and I don't want to take away somebody else's ME time.
Somebody who still needs it.
I haven't flown a SE or any piston for that matter in 3.5 years.
I'm slowly starting to think about doing a flight here of there with friends that still fly GA.