Tank, some planes really need 100%. All 109s G and up need 100% internal. That gives 25 minutes on the K-4, 20 when you hit WEP (of which 10 minutes can be eaten in WEP, remember). The F4F/fm2 only has a 30 minute full tank. F6F seems equally short legged. I never up a P-39 with less than 100%, often 100%/DT. Very short duration on that one. P-40E I like 50%/DT or 75% clean, depending on situation. 100% is 40 minutes and just a tad too heavy to fight well.
I agree on the Brew though, too much gas to fight with more than 50% effectively. Most non-N jugs I fly I take 100% to get the aux tank. It means I burn more gas on purpose to lighten up a bit (maybe take off a field away from the fight, etc) but I manually keep half or 1/3 the AUX so I can make it home if/when my main gets a leak. It used to be a given your first hit would be a main gas leak. Seems to have stopped in recent years, though. I still do it from habit.
Planes I almost always up full fuel (usually shorter ranged planes)
109g14/g6/k4
spit9/8/16/14
c205
all jugs except N
Corsair f4u-1c/1d/-4
f4f4/fm2
Either P-39
typhie/temp
190a5
I-16
La5/La7
Yak-9T/U
It's a matter of duration. These all have flight times around 30 minutes or less with full internal fuel. I plan to be able to get into more than just 1 fight in a sortie, so I want to be able to burn wep a few times, dive, climb, egress, ingress, and then rtb. I usually want 20 mins bare minimum, if not 25-30.