I like to warm up for 15-20 minutes before any real climbing. When I leave from home, however, there is this steep, nasty thing and I go to my lowest gear (30x25) and do switchbacks across the road so my heart doesn't go from rest to max in 5 seconds. I also like to think it's better for my 53 year old knees that are carrying 205#.
Some people complain that a long stop in the middle of a ride makes it too hard to get started again. I don't think that's a problem, I just start slow as if the ride had just started. We ride on Angeles Crest Hwy and it has a series of 6-7 mile climbs followed by 6-7 mile descents. Let me tell you, if you've been climbing all day and you finish a 7 mile downhill, your legs will scream at you on the start of the next climb!