The only time I take all the ammo is if I'm on an attack run and will be strafing. I almost always run out of fuel before ammo. You save some weight from the reduction, which I find to make a big difference (a little snappier roll rate), and worth about 1.5" of manifold pressure in climb.
For fighter missions:
In the D11, I always take 100% fuel, sometimes even the drop tank. In the D-25 and D-40, almost always 100% but without the drop tank (carries a bit more fuel internal). In the N, almost always take 75% with no drops. I then burn the wing tanks off as soon as I lift, and know I'll have (what is 100% fuel in the D-25/40) full fuel in the main and aux.
But, the key is being able to be in the fight once you've only got 1/4 of a tank of fuel, and 600-800 rounds of caliber .50 left. The plane begins to get downright sporty as it approaches the 11,000 to 12,000 pound range. I've got no problems mixing it up with almost any aircraft once I get good and light--say with 5 minutes or so left on station time.