I think I disagree with some of the comments.. For a simple safety stop, just relax and breath normally.
When I do a deco stop for more than 4-5 min. I do something different. After 4-5 minutes of hanging at a deco stop my breathing rate and metabolism is pretty slow. I deliberately try to do short periods of exercise during deco to improve blood flow to the extremeties.. I move shoulders, elbows and also do periods of light swimming until i feel the slightest onset of fatigue in my thighs. Then resume total resting.
This activity improves blood flow, naturally increases respiration and heart rate.. We have some anecdotal evidence that not moving your arms and keeping the elbow bent during the entire deco can cause problems. I even try to switch hands if I am holding something and try to make sure my hands and wrists get a little movement and are not fixed clutching something during the entire deco.
I do a lot of those things because I get bored. Good discussion on this last year: http://www.scubaboard.com/forums/archive/index.php/t-322301.html
Short answer was that mild exercise may have benefits, but artificially altering your breath patterns has more negative effects then potential benefits.
Matt