Personally, I would say if you're on crutches, never exit by the rear door of a bus. First of all, they're usually narrower than the front doors, secondly, the front door is more likely to be unobstructed when the bus stops, and thirdly, if you fall, the bus driver is much more likely to see you if you exit at the front.

I'd also say mind your armpits, because you're likely to get chafed, but I don't know whether that's a safety tip or not.

Other than that, I got nuthin', because I actually really can't walk on crutches. (My balance is that bad.) I can handle walking unassisted, or with a cane, but I tip backwards on crutches.