Looks like this topic will be beaten to death here, as well.

Assuming you have normal body proportions, a bike is more or less balanced fore and aft when riding unloaded. Most bike designs aim for a roughly 40/60 f/r weight distribution. Any weight added to the front OR rear will unbalance the bike from its designed weight distribution. Most people find handling less adversly impacted by adding extra load to the rear first, and then rebalancing by adding load to the front.