I wouldn't stress a few extra inches of #6 wire to make for better mounting. 1000' of #6 wire has a resistance 0.3951 ohms. Even at a 50 amp draw, 6' of #6 gauge wire has less than 0.25 volts drop. I'm not saying be wasteful by any means, just that if you need an extra inch or two to make a better robot, it's worth taking it. Go here to play with values to see what your losses will be:
http://genuinedealz.com/voltage-drop.html
-Mike