Anytime you want to increase flexibility or decrease tightness... stretching is a good option. However, as others posted above, it is best to stretch only after a proper warm up.
Stretching AS your warm up = not good.
Stretching without a warm up = not good.
Stretching to a point of pain = not good.
I like to stretch after my workout. I hold each stretch for 30 seconds. I use this as my cool down. Look into some yoga site (yogajournal.com) for some good poses.
I also use a lot of basic stretches (the kind from gym class!)