For many years, I didn't bother with warming up before going into my routine. Nowadays, I warm up before and heavy lifting. I have found that this actually helps me to lift heavier weights in my work sets.
Some muscle groups I warm up more than others. For example, I warm up delts with a couple of high rep, low weight front and side delt raises. I started doing this because I was having a lot of pain in my left shoulder which inhibited my lifting. Before bench pressing, I warm up by doing lighter weight incline bench dumbbell presses. When working quads, I warm up by doing leg extensions prior to leg presses and hack squats. I never have any knee problems. Most warm up sets are at least 12 repetitions. For many exercises I'll do 12, 10, 8 and 8, adding weight with each set. When the weight is light, I concentrate on using the best form I possibly can and a very even pace....no jerking.