I think it was around the early-mid 90's when kicking out of finishers slowly started to become more common. Before then, kicking out of a finisher was done virtually never at all, at least in the WWF.
I don't mind when it happens in big match situations like WM25 (Taker/HBK) or the WM28 HIAC match, but even then, when they do it three or more times in a row, it grates on my nerves. I think I probably never mind when someone escapes from a submission hold by grabbing the ropes since the ropes are kinda there for that purpose, and all you have to do is reach for them. But a physically damaging finisher (Tombstone, Pedigree, Stunner, etc.) should always end the match at least 95% of the time after being done only once, unless the person who executed the move is just too slow to make the cover (giving the other guy time to recover.)