If the front end bottoms out on a jump should I increase or decrease the pre-load on the front shocks. This is the only adjustment I have on them.

 

Increasing the preload makes the shocks stiffer and lessening it makes then softer. I would say that you should add more preload to stiffen them up and reduce bottoming out.

 

Preload is not meant to compensate for valving, it is meant for setting rider/bike weight and sag and will most likely have little effect on bottoming and significantly affect ride harshness.
If you are having a problem bottoming, send them off to be revalved (if they're rebuildable) or save some money and do it yourself if you have the proper tools. If not, check to see if TCS can do anything for you.

More than likely there is a vast improvement to be made over stock.