Ben:
The best advice I ever got on the subject was, "If you are thinking about dumping the water, DUMP IT!" And that has remained pretty good advice.

The question you have to ask yourself is "why am I thinking about dumping water?" Is it because the last few thermals were weaker? (Dump it). Or, "I am perilously low and I need to float to a save?" (Dump it) Or, "The guys without water are seriously out climbing me and running away" (you guessed it - start dumping).

I think the only time to hold on to the water (when doubts emerge about its helpfulness) is when you are reasonably certain that you will get back to better conditions. This may be because you have started early in the morning before it cooks up or when you have drifted down to the weak area of the height band and can climb back up to the strong lift. But it's not a science - it's an educated guess about the future.

ROY