The best time to stretch is after the body is warmed
up, relaxed, and when the blood is moving. Since
many runners do stretch incorrectly, it's best to wait
and stretch after warming up. Don't stretch to warm
the muscles up; it won't work. Stretch in the evening,
for example, or throughout the day as you have time.
Many of my friends use stretching as a nice way to
prepare for sleep.