When recording, is it better to turn Correlate all variables to on or off?
It depends on the variable. Don't correlate anything that you want to stay the same, don't correlate anything you want to datapool. Do correlate session identification data. I usually generate the script once with no correlation, once with everything correlated, find the names of the variables that I actually want correlated, then list them in the "correlate specific" box.
See User Experience, not Metrics part 12 on http://www.perftestplus.com It is about different ways to correlate data. Not exactly your question, but it might shed some light on things for you.
Scott Barber, Sr. Performance Engineer
That sounds like a good idea. Thanks!