Well, if you are going to use it exclusively as a varmint rifle - a 1:14 is fine. Most folks, even if they didn't originally intend to engage big game with the rifle, eventually want to try this. I think they start to develop so much confidence in their ability to hit distant targets that they want to hunt everything with their 6mm-284's.

A 1:10 or 1:9 is what you need for the heavy bullets. They still tend to shoot the 70 grainers pretty well with a faster twist - although pressure tends to build a little quicker. Some elect to split the difference and go with a 12" twist. This will allow you to stabilize bullets up to the 90 grain Ballistic Tip. A lot of these folks use the Sierra 85 gr. HPBTGameking, at about 3700 fps, for everything.