Over at Tenkar's Tavern a couple days ago, Erik Tenkar asked how many people roll their wandering monsters in advance, and what people thought about it. I don't think I ever have, but I don't feel it really matters, since rolling encounters in advance still remains true to the fundamental principle behind rolling random encounters at all.
Why roll for an encounter at all? We often talk about randomness as a good source of inspiration, or of the GM enjoying the surprise of it all, just as much as the players enjoy being surprised. That's certainly part of it, but the core reason is that random rolls take the decision out of the GM's hands. A wandering monster roll is, at its heart, about fairness. The players must be made to feel the pressure of time and the suspense of not knowing what may come around the corner or through the door, or when. But randomly rolling to see if a monster wanders by, randomly rolling which monster wanders by, and randomly rolling how many monsters show up, keeps the whole process fair. It is not the GM deciding to slam the the players with large roving bands of tough monsters, or frequent nuisance encounters,, nor does the GM decide to take it easy on the players because of reasons that have nothing to do with what the characters are doing in the game world. It is the dice.
Rolling in advance actually supports this a bit. If you roll in advance, you have no idea how many characters will actually be in the party, or what preparations they have made. You aren't adjusting the encounters or the frequency based on metagame desires, because that knowledge doesn't even exist yet. Anything could happen before you get around to actually using the pre-rolled encounters.
Rolling during play doesn't change this, but you might be tempted to fudge the roll based on what you know now. Of course, you might be tempted to fudge what you wrote down before play began, too, but at least there's a slight feeling of "but I already decided it would be this."
The die is cast, and the fates have decided.