Depends on what you are going to do with your truck off road. If you are just worried about being stuck in snow then a 8 or 9000 lb winch will be fine. I have an 8k Multi mount and it has always got the job done.
Yes the "rule of thumb" is 1. 5 times the weight of your rig, but I think that is for serious wheelers who rock climb and mud bog.
Light duty stuff like snow, ditches, etc a smaller winch will suffice. I would reccomend a Warn 9500. That is what I will buy when my 8000 takes a crap, but it has been in service for 12 years now and no problems. It does not get used heavliy though.
Just my . 02