WARM-UP / COOL-DOWN
The warm-up
For normal slow/steady/medium jogs it is enough if you run very slowly for approx. 5-10 min, then take a break of approx. 5 min for stretching. Only hold the stretches for 8-12 seconds in the warm-up phase and only stretch lightly, so that the muscle tissue is not overstrained. 2-3 stretches is enough for each muscle group.
Before intensive running (fast jogging, timed jogs, timed runs) and particularly before a competition, run for 10 - 20 min (depending on the intensity), followed by 5 min of stretching. After this, do a few interval runs, gradually increasing speed. Finally, jog slowly for 5 min (this is better than leaving the stretching exercises until the end of the warm-up phase).
Before a competition, you should have another 10-15 min break before the starting gun goes off.
Before a marathon, jog for just 5 min, do 5 min. of stretching, and then run 2 light intervals for another 5 min., gradually increasing in speed.
The cool-down
With normal jogs, it is enough if you run the last kilometre (or the last few minutes) even more slowly.
After intensive exercise and competitions you should run slowly for 10-15 min, as the body needs this time to wind down. Follow your cool-down with 10 min of stretching exercises (even after normal jogging!), holding the stretches for 20-25 seconds and not pushing too hard.
If the training/competition was so hard that you don't feel like stretching at all, then leave it out. But don't make skipping stretching into a habit!