I am sure there are a lot of things that are affected by weight distribution - such as wear/stress on hubs/wheels, frame, etc. However, I would think the most important aspect would be to get the handling of the bike as stable as it can be. When I started doing my long day tours I put most of my gear in a backpack and strapped it to the rear rack. It worked for a while, but it was annoying and made the bike top heavy. When I dropped the gear off the sides with kayak bags as panniers the handing was vastly improved. The front end is pretty light going up hills but I haven't had any other stability issues. For a multi-day tour I'll expand with gear on the front. I am not particularly concerned with "weight distribution", however. My tires get rotated every once in a while to balance out tread wear.