Well, the oil in the lines may well trickle back, but when the level in the tank has visibly dropped I'd say it's coming from there! Haven't done a tap like Ronnierockets on my A, but have 2 similar (not as pretty) on AMC twins, and can confirm that with them fitted, no 'wet starts'. (Plus no excess oil in the primary drive from the lack of drive side mainshaft oil seals on those particular engines, which makes a full crankcase worse to deal with.)
What's funny is that now, with a couple of bikes with taps and others without, I'm paranoid whenever I start anything and look automatically for taps that aren't there on things that don't have them.
So the mental discipline does set in, which makes a catastrophe less likely, even if the switch fails because the carbon brush doesn't make good contact, etc. Cut-outs can't be 100% reliable for ever, without regular checks from brush to button. I have been thinking about a way of making the kickstart swing force the lever on the taps open in the event of mental lapse . . . . but not so easy to arrange.
The AMC sites similar to this one are quite full of tales of expensive woe from folk who forgot or otherwise made a horlicks, so the mental thing really does matter. The biggest risk moment is probably on starting up in the middle of a day if the tap has been switched off at a halt . . . best therefore to open it the once and leave open till end of playtime.
