As a DM, absolutely. If the players have fought a long, dramatic, cool battle against a very tough opponent, players are low on resources and HPs, as is the opponent, and the 1-HP baddie who can't escape lucks out on a crit with a x3 crit multiplier weapon, the crit didn't happen. He hits, PC takes normal damage, maybe knocked down into low negs, but he'll live long enough for his fellow PCs to settle the bad guy's hash and apply some kind of healing to the PC.
Unless, the player wants his PC to die in a dramatic way. Then I'll allow some flavor speech on the PC's part and then roll my crit damage.
As a player, no. Well, let me amend: I try not to. I try to have complex arithmatic done ahead of time, like attack rolls in light of Combat Reflexes or Power Attack. But sometimes, I make a mistake. I'll own up to it and the DM can backtrack if he wishes.