For the trico hatch on a tailwater I fish, I use 6x and haven't had any issues, my poor slackwater casting aside. The fish get pounded daily and really don't care much for drag, but the river is full of weeds (and very large trout) and with 7x I'd just be leaving flies in the fish's mouths or wrapped in the salad (a number end up donated to the ecosystem as it is anyway); heck, none of the 3 shops in town carry anything smaller than 6x. Can't say I ever left that water thinking "if I only had spent the $4.50 for a spool of 7x I would have slayed them", more like "if only I'd spent some time practicing my casting, I might have actually put a fish on the reel".

Regards,
Scott