Fred,
best glide ratio is attained at the speed that gives the best Cl/Cd ratio.
This is a constant for a given wing/fuselage combination. Therefore, the speed you need to fly to get optimum glide depends only on weight (as W = L = Cl x 1/2 p v^2 x S, assuming constant air density).
This means that at higher weight, the best glide speed is also higher, but that the range remains the same. The goal in competition is to fly a given distance at the highest speed, so that it makes sense to take on water ballast.
Erik.
Edit: apparently I need to improve the rate at which I type
