I'm no expert, because I've just recently begun mining with my GPUs, but I've been keeping my cards very cool (65c or cooler). I do not run my cards at 100% TDP, something closer to 75% and I leave the fans to auto. Depending upon the design of the fan, leaving it on 100% fan-speed may or may not significantly affect the life of your GPU.

I would monitor the temperature of your cards and prioritize that, because from there you will be able to tell if you need more fan speed. It depends on your cards, your rig, your TDP and other settings; but my biggest concern is keeping the cards cool and prolonging their lifespan as much as possible. This leads me to use whatever fan speed keeps my cards below 65c, Auto has worked well and normally keeps fan speeds below 40%. You don't want to overwork your fans for no benefit.