One dilema about having 1/4-1/2 is the sloshing around of the gas in the tank during your starts... it may not sound like a big deal but liquid is heavy and you can think of it as having a few bowling balls in your trunk.

This is a THEORY but it is very understandable and has been tested... gas moving around like that can really throw off the balance of the car so you may see quicker times with more or less gas regardless of traction.