Here is a link to a reprint from the Daysailer Association's quarterly newsletter regarding setup and rig tuning.
http://www.bobhunkins.com/rntds.PDF
I think that aside from using a tension gauge, that using the leeward shroud showing slack at 8-10 kn of breeze is a good setup for proper tension. Some top racers tension up quite high but then release the tension at the end of each day to avoid distorting their boats. Those people tend to run the same tension regardless of conditions where as other top racers will adjust the tension for conditions where they go for higher tensions with heavier breezes.
I like to run higher tensions when heavy weather sailing and sometimes do that while underway. If the leeward shroud is going too slack because of high winds I'll give it a couple of turns on the leeward turnbuckle and on the next tack even it out. I find if I am using more mast bend and vang tension that I also like more shroud tension.