In theory they should reduce
induced drag - by inhibiting the formation of wing-tip vortices. Just like winglets or other wingtip devices, they would tend to block spanwise flow of high-pressure air beneath the airfoil around the wingtip into the low-pressure region on top.
http://history.nasa.gov/SP-367/f59.htm
You'd have to do calculations for a given size and shape of tank to determine if the reduction in induced drag (if any) would exceed the additional form drag from the tank itself.
Additionally, tip tanks allowed the use of a thinner and more efficient airfoil in the case of the Cessna 310 series, since no fuel volume needed to be accomodated within the wings themselves.
It's worth noting that Cessna didn't use tip tanks just to be clever. Within the dimensional constraints set down early on, the wingspan was limited. Cessna's engineers also didn't want fuel between the engine nacelles and the cabin — not that there'd be a lot of room anyway, with the hefty main gear swinging inboard. Without making the 230-series airfoils unacceptably thick, there just wasn't enough volume in the wing outboard of the nacelles to carry the desired amount of fuel.
- from
Classic Cessna 310 Ariticles
The same applied to the F-104 - the knife-edge wing needed for supersonic flight had no space for fuel (or landing gear, for that matter), leaving fuselage or external tanks as the options.
http://www.generationv.co.uk/ejcgall.../f104_2_3v.jpg
You'll note that tip tanks tend to appear on planes with short wings (note the mention of limited wingspan in the Cessna quote) - and short wings benefit more from tip devices (and require less structural strengthening to carry fuel weight at the tips - less lever arm).
The B-47 had a thin wing like the F-104, but much higher aspect ratio. Thus wing tip tanks (and the suspended weight) would have caused structural problems, while producing less benefit from vortex reduction (already improved through the long aspect ratio of the B-47 wing). So the B-47 design used mid-wing external tanks and fuselage tanks.
http://www.aviastar.org/pictures/usa/boeing_b-47.gif