I usually go ahead and pitch the whole packet, vial, or smack pack. For any recipe with an OG over 1.050 you'll need a full package of liquid anyway. I've never been too confident on keeping a pack of dry yeast after opening it and I hate wasting them so I just chuck the whole thing in.
This also assumes you are talking about ales. You'll need about twice as much yeast for a lager. Check out the Mr Malty calculator for more on pitching rates.
I usually only make 2.5 gallon batches, and I always throw in the full packet of yeast. Dry yeast is cheap, so you might as well. Especially if it is a high gravity beer, the more yeast the better.