In the cold its definately worth stretching the vitals, calves,hams,back etc i spend 10-15 mins stretching whilst im getting changed, its just become part of my routine now.
The theory goes that by stretching without warming up first, you actually increase the chance of injury, when you train.
When people talk about warming up the muscles to prepare them for the physical effort ahead - this refers to increasing the temperature of your muscles by using your muscles in using little effort. So, if you are going for a run, you could do squats, using only your own bodyweight and standing hamstrink curls (in your warm room); then, when you get on your running track, you might start off by simply walking and gradually increasing the intensity.
You should not be stretching (note that stretching and warming up are 2 very different things) while your muscles are cold, but should be lightly moving the body parts you intend to be using.
You must also take into account any previous injuries you have had, as injuries never recover 100%. For example, my shoulders have had a lot of injuries, so before I do any upper body weight training, I always warm up my shoulders for 5 minutes, using a 4kg weight. Note that I do no stretching during this warm up, as this will increase the chances of me injuring my shoulders.
Once you have added heat to your muscles, you can stretch till your hearts content, though don't immediately go into a deep stretch - start of with a relaxed stretch and if you want to increase your flexibility, then gradually perform deeper stretches. The best time to do this might be in the middle or end of your run, when your muscles will be warmed up.
In the 80s (and earlier), lots of people used to incorporate stretching as part of their warm up routines. In fact, some newbies of today still do this, but newer research suggests that this is very bad. This is why top class athletes, for example, in the 21st century will add heat to their muscles BEFORE stretching the muscle group and won't perform stretches on cold muscles.
I find I need to leave about an hour between eating and going for a run. I run around the common and it's about a 5 min walk to it from my flat so i just walk that to warmup and then do my couple of laps, walk home, stretch when I get back to the flat and then think about some food.
Thats a good warm-up and stretch routine.