No, you don't need a torque wrench. But you do want to get it as tight as you can get it. Here's what I recommend. First off, if you have one of those 4-way combo wrenches that will also remove glow plugs get a good grip on it with your right and and throw it as far as you can!!! Seriously, those are no good for tightening a prop. What you do need is a good box end wrench and a glove (a good rag will do as well). Use the glove to hold the prop while you tighten it down, this will keep the prop from cutting your hand. Don't worry about over tightening because the nut and shaft are steel and will be very difficult to damage. Pull the wrench just as tight as you can get it. If get it nice and tight now you won't have to worry about ever throwing a prop. It's been at least 6 years since I've thrown a prop.

Ken