I always spell out numbers -- in both action and dialogue. But I think it's especially important in dialogue, for a couple of reasons. First, it helps with the whole "1 page=1 minute" thing. But more important, it takes the guesswork out for the actors. Say you write in dialogue an apartment number -- 1415. Do you want your character to say "one-four-one-five"? Or "fourteen-fifteen"? It might be important in some cases. I always try to write it the way I envision the character saying the words.