I flew JP5s for a while. Some were fitted with tip tanks, others not. The tip tank equipped aircraft went further on the same fuel than the "bare winged" ones, or in the job I was trained for, the tip tanked aircraft flew the same speed at lower engine power. In other words, the tips provided an aerodynamic benefit.
But spinning with fuel in the tips wasn't allowed. It upset the B/A ratios and all that stuff.