I don't know about the B model, but the D model had an extra tank behind the pilot (I believe that IS the AUX tank,) and with it loaded with fuel, the normally delightfully stable P-51 was a bloody deathtrap. Given the ranges it was meant to fly, however, this wasn't much of a problem- the pilots would usually empty it to climb to altitude before joining their bombers.
A lot of the LAWL MUSTANG SUX comments come from people who take up a 'Stang with 100% internal fuel, and expect it to maneuver like a 109 or Spit, which both have about a single hour of internal fuel, compared to the Mustang's SIX. (Eight with drop-tanks IIRC.) Fuel loading/burnoff is definitely important for a Mustang- and the P-47, for that matter. The technical manual for the P-51B specifically says to empty that bloody AUX tank first. If burning it to 60% floats your boat, I guess you can go for it, but I'd rather have it empty.
On the other hand I don't fly Mustangs as often, so I'm basing this mostly off what the real pilots did; which doesn't always translate to AH well (understatement of the week.)