Here's a short article on fluid loss and exercise from the University of North Carolina Medical School:

http://www.med.unc.edu/...gold/fluidintake.htm

Here are a couple of excerps:

"...athletic performance will begin to suffer after a loss of just 2-3% of body weight. After losing 5% it is common for a person to begin feeling nauseous, dizzy, or sick."

"After losing 7-10% of your body weight a person is in serious danger and may need hospitalization."

Richard

the answer is fairly simple. the rule of thumb is that you should allow for 0% of fluid loss!
5lbs is indeed a lot, specially when considering that the vast majority of us are already dehydrated at the start of a training session...
It is actually quite rare to see people keeping their hydration status where it should be on a regular basis, making sure that they maintain an optimal hydration status throughout the day, training or not training...