It isn't good to stretch before a strength training routine, since it can make your muscles contract and become shorter, and tighter muscles aren't exactly what you want when working out. Instead of stretching before you head to the weight room, warm up with five minutes of jogging, jumping jacks, elliptical, or other quick cardio routines to get your heart rate up.
Stretching, however, is important for staying flexible and helping to prevent injury. So stretch at the end of every exercise session, after your muscles are warm. Or, try this time-saving tip: stretch in between sets to cut down on your cooldown time.