I learned from my dog's getting sick last summer (which at least made me miss the joys of a snowstorm at 11,000 feet) that it's a good idea to build at least one day off into your trip, maybe more. This will allow you a rest day if you're not feeling up to par or just want to hang around a beautiful place longer or want to dayhike to explore something. It will also allow you to hunker down in bad weather (such as above-mentioned snowstorm) or allow a blister to heal or (in my case) nurse the dog back to health so the trip can continue. Of course if none of these contingencies occur and nothing along your trip seems worth an extra day of either exploration or R&R--well, if the trip was that bad you can go out a day early!

Flexibility in planning is a good thing, IMHO!