If you wait on players to roll every time, we will *always* have to wait for the slowest poster (which can be days, which by itself is not really a problem, if someone cannot check in for a couple days, he/she will simply catch up afterwards, no problem!), and in the meantime *everything* is halted.
It would also be wrong, if only 1 or 2 players are posting extremely fast, to match the speed to them, but a good average is probably the best thing to strive for.
If 4-5 of 6 players are posting, and 1 or 2 just happen to have to struggle with free time right now (happens to everyone here and there, so don't worry about that! If someone doesn't post for weeks, it's time to start worrying

), I don't see why the game should completely stop then. Just move on. A decent speed is crucial for online games, I believe.
That's why I think it's much better if the GM makes the rolls, or just makes the rolls, that do not happen after a reasonable time, if asking the players for rolls (which is certainly fine, always nice to have this feeling of having tossed the dice yourself (even if it's only an online dice roller, heh)

).
Secret rolls (like when players should not be alarmed that something is wrong by the fact alone, that a roll is made) should always be done by the GM, anyways, and the results only posted, if there was a success.
That's, at least, my take on it... comments are welcome.

Bye
Thanee