I feel it is crucial and load my plane accordingly.
My standard load-out for any frame is 25-50% fuel and a DT. I usually take the heavy ammo loads, but go with the fewer guns packages...
And from experience in the DA & KOTH, there is a huge performance difference in fuel and a marked difference for ammo loads. As the others mentioned above, it differs according to airframe.
Also,
Tank burn order is crucial in some rides.... (F4U, 190s, and 205)...