I have one, and use it on occasion. The only times I ever use it are when I install my mtb's bb or cranks. The cranks on my fg are steel, so I'm not as worried about stress cracking them as with my aluminum ones. It's good to have if you do a lot of service on your bike, i.e. removing cranks often for travel etc, so that you don't repeatedly over tighten, but you don't need one if you just use common sense. For instance, you don't simply finger tighten the bolt that holds your cranks on, but you also don't jump up and down on the wrench trying to get it as tight as humanly possible. Use common-sense and you will be fine without one.

I also find that after installing a crank it's a good idea to check the crank arm bolt's tightness after a couple rides to make sure it didn't break loose.