I seem to remember America West 757s, notably Teamwork cls, that were hard to get the reg's visible, same goes with TransMeridian 727s currently and certain 777s where you'd have to be just aft of center.
I think that sometimes, when shooting slides side on/gear split, etc., the reg. can be blocked partially but only when the color scheme is a one off, like Teamwork was, the Arizona Cardinals 757 or a Disney 747. If it's a common airliner, like a Continental MD
-80, where the reg. is down low, then you'd have to take it slightly forward or aft...I prefer forward of course!