I think, its very dependent on the opponent type, board runout, action and of course also the stakes. If a nitty microstakes regular raise you on the river after calling preflop, calling flop and calling turn, its almost always the nuts, like Blackrain79 talk about in his first book. Or even just a tripple barrel from them especially in a 3-bet pot. Basically any line, where they put their 100+BB stack at risk.
There are other situations however, like if the river card is a complete brick, and now a recreational player lead into you after calling the previous streets. Yeah maybe sometimes they rivered two pair with their K3 or whatever, but often they missed, whatever they were chasing, and now they want you to fold. So in general, if the story makes no sense, we should call with most of our bluffcatchers, even in the micros.