i warm up all the time, tho sometimes you cant cos your late but i believe you should do it as much as possible. it allows the oils to circulate, and limit engine wear.

on a cold start my car normally revs up to about 1600rpm and i take off when it drops down to 1000rpm and because the temperature needle in the morning is below the cold line, at 1000rpm it sits right on it though i still dont fang the car until it gets to operating temp.