Many studies have looked at the effects of static (traditional) stretching on both endurance and strength gains.  Here are some interesting facts to consider:
-Stretching longer than 60 seconds before a heavy workout set drastically reduces maximum strength. Some studies show stretching as little as 10-20 seconds can reduce strength by 5-30% according to the muscle group being stretched.
– A study looking at female long-distance runners found that stretching before a long distance run had no effects (positive or negative) on their run-times or level of endurance, however their flexibility did improve.
– Another study found 4 bouts of 1 minute of hamstring stretches 3x a week improved flexibility by 30%. This was done over a period of 8 weeks. So even stretching a muscle group 3x a week is sufficient for drastic gains in flexibility.
– Static stretching performed post-workout has been shown to reduce DOMS (delayed onset muscle soreness), the soreness you have 1-2 days after an intense workout.
Conclusion:
While stretching is extremely important for improving flexibility it would best benefit those who are performing resistance training to hold off until the cool-down phase of their workout. Any person about to undergo a long distance run or cycling-bout could perform flexibility stretching in the warm-up or cool-down, however, if they are in a race where sprints would be performed (or doing intervals) they may want to hold off until the cool-down as there would be a decrease in performance similar to stretching to warming-up for resistance training.
When trying to improve flexibility a stretching routine of 4 bouts of 1 minute per muscle group performed 3x a week can provide up to a 30% increase in flexibility, over an 8-week period.