I don’t think you need to stretch these muscles too often but it’s good to stretch after using them, especially heavy biceps muscle days. I see some guys who have very tight biceps muscles and can’t extend their elbows fully. They really need to stretch biceps muscle otherwise the opposite muscle (triceps) is lengthened and can’t generate as much power as possible when the triceps are in the normal range.
When you should stretch
There is some controversy regarding stretching as part of the warm up, although if very high intensity elements are to be included in the workout, stretching may actually inhibit the ability to achieve full intensity.
This is attributed to the fact that stretching improves muscle elasticity which lowers the force generating capacity of the contractile proteins of the muscle. Moreover, the practice of stretching before performing any warm up is not justified and may potentially be harmful.
Triceps muscle stretch
I performed this in a standing position but you can do it while sitting on a chair. As you see in the picture, pull one hand with the other hand by using a towel. Hold the position for at least 30 seconds and do it on the other side as well.
Biceps muscle stretch
Find a surface that will be higher than your body when you kneel down, place one hand, thumb down, then slowly kneel down. Stretching should never be too quick or bouncing. Again hold the position for 30 seconds and do it on the other side.
Click here to learn some mobility / stretch exercises
Some exercises during personal training