You don't want to run your fans at 100%. You will burn them out quite quickly. They are not designed to handle that. You shouldn't have to run your fans greater than 70% to stay in safe operating ranges. 60-65% should be more than enough. If you do have to run them that fast you are pushing your cards too hard or your ambient temp is too high.
I would love to see some empirical evidence showing that running a fan at it's rated voltage continuously shortens its life vs running it at @ rated voltage but controlling speed using PWM.