What I am about to state is my understanding based on reading a large number of posts by bagginsbill and others. I may be wrong and will gladly be corrected... but this is how I understand it.

The number of raytrace bounces that will be used depends on the number of reflective/transparent/translucent surfaces in the render. 

Given that, once the needed number of bounces occur, it become irrelavant what you have the bounces set to. Try it yourself. Set up a render with a mirror in it. Set bounces at 4. Render, using the timer function in the d3d script. Now increase bounces to 20. Render again. What result do you get? Should be the same.

I leave my bounces set at 5, because I don't ever set up scenes that need more. It does not seem to impact my simple scene renders, at least not so that I notice it.

If BB reads this, I hope he will comment to either verify or correct what I'm saying here. I THINK I'm right.