I'm late to the party here, but I do have experience with helixes in a couple of scales. Things to be careful about are spacing between levels (which in turn determines the grade) and length of run (and thus time the train is in the helix).
In your case, an O72 helix will have a run of 226 inches. For 6 inches of separation (railhead to railhead), the grade works out to 2.65% which is not too bad. It will seem like a 3% grade or even slightly more to the locomotives, since the entire train will be on the O72 curve. The 6 inches is top of rail to top of rail. If you use 1/2" plywood with Gargraves, Ross or Atlas track on top, the clearance from top of rail to bottom of plywood overhead is more like 5 inches.
A 4-turn O72 helix has a run of 904.8 inches or almost 75 and a half feet. At 10 to 20 scale mph (good helix speed), a train should take between 2 and 4 minutes to run the helix (again not too bad for a train waiting its turn).
