I always wait (if I can) until RPM is 1 or so. Especially in the winter.
My cousin has 2000 Accord and he says that according to manual you shouldn't warm it up even in the coldest days.
One of My theory was to warm up (for some cars) is compressions. Like my Vigor has high compression ratio, and if its colder enigne it will not perform as it should. For other cars it don't matter.