Well when I started playing with the RC models and specifically the gassers, I put the loops on top of my tanks because that's the way everyone around here did the gas engine installs. When I put my Comp-Arf together it was much more convenient to just drop the vent line out behind the fuel tank. Trust me, there was fuel all over the belly when I landed and so I put the loop in the vent line and that siphoning went away.

So what can I say? Anecdotal evidence? Sure but it worked for me and the only models I have with smoke systems are the turbines with composite fuselages which are painted inside so when that d*mn smoke oil escapes, there isn't any exposed wood for it to soak into.....