warm up before and get things loosened up.. but don't do deep stretching before whether you are lifting or running.
stretch after.
football players stretch before and they are always pulling muscles... 'but steve then why do they stretch?' they stretch cause they are getting bent in awkward directions, so if they don't stretch they run a higher chance of tearing ligaments, tendons, and muscles. unless you are gonna bent around like that then its not a good idea to stretch beforehand unless its very very light stretching.. wait till you are finished then stretch away.