The most weight goes on front feet. So front feet are more "sensitive" to the stones, wearing, etc. etc. etc. Being white or black doesn't matter actually - horse has good feet or bad feet. :) Also shoes adds friction (?).
People who ride on rocky terrain usually shoe all 4. I know jumpers and barrels also do the same. In my area (not as rocky) most people shoe just front feet. I personally go with boots. :)