I would urge you instead of topping off to reach an arbitrary VOLUME, like the 5 gallons, to top off to hit your recipe's original gravity (OG). The kit instructions to top off to exactly 5 gallons assumes that everything goes perfectly throughout the brew process, and it rarely does. Not getting 100% of the extract out of the container/bag, not hitting steeping temp perfectly, boilovers, not siphoning 100% of the wort, etc. can all affect your REAL final volume. Hitting the OG is much more important to end characteristics and quality of the beer.
After your cooldown, take a gravity reading before you add any top off water at all. This reading will be HIGHER than your expected OG. Then, slowly start to add top off water, stirring it LIKE CRAZY. Gravity is a measure of density (relative to water), so if you don't mix the water thoroughly, you won't get an accurate gravity reading. The oxygen you produce while stirring is also very good for yeast health once you pitch your yeast.
When you are 1/2 gallon or so from the 5 gallon mark, take another gravity reading. It is not uncommon for you to hit your OG a good 0.2-0.5 gallons BEFORE the 5 gallon mark, but it is better to come up short and have the proper OG than it is to go PAST the OG and have watered down beer.
Anyway, stop adding top off water when you hit your OG reading, which may or may not be the full 5 gallons. Your beer will be better for it.
