I think the main questions here are:
- What is your speed vs. the stall speed of your aircraft?
- What is your altitude?
- What is the visual acuity of your girlfriend? (In other words, how obvious do your waggles need to be for her to notice?)
If you want to fly a circuit above someones house in your average spamcan, my suggestion would be to maintain 1000' at least (not just for legal reasons) and to maintain something like a reasonable cruising speed, or the speed you fly on downwind (80-90 knots in your average spamcan with a stall speed of about 50 knots).
This gives you enough performance to do a 45 degree steep turn, and this also gives you a reasonable buffer above the stall for a simple wing rock. So a wing rock of up to 20-30 degrees either side, even if a bit out of balance, will not cause any problems. But most likely people doing this will do this in your average spamcan (C172, PA-28 or similar machines) which have the adverse yaw mostly tweaked out of them by things like frise ailerons. So even if you rock your wings rapidly without doing anything with your feet, the aircraft will yaw a bit in the proper direction and keep you in balance. More or less.
So this should be not much safer or more dangerous than doing steep turns up to 45 degrees. (Oh, and of course you remember to add power during steep turns, didn't you? Might be a good idea to add a few hundred RPM when doing wing waggles too.)
But a wing rock like that will look sloppy from the ground because the tail wags as well. The question from the OP was how to prevent that, so that a wing waggle actually becomes an axial roll. For this you need to cross control: a waggle left needs a bit of right rudder. How much depends on the airplane involved, and to what extent the adverse yaw has been aerodynamically tweaked out. Waggles of 45 degrees will also temporarily get you in a sideslip which you may or may not want to counter with a bit of knife-edge flight techniques - which means more opposite rudder. It also need a bit of pushing otherwise your lift vector will take you off course. And all this cross controlling, in addition to being rather uncomfortable for passengers, will rob you of somewhere between 10 and 20 knots of speed - depending on your entry speed, even with full throttle.
So if you're flying in a spamcan with a comfortable speed above the stall (80 vs. 50 knots for instance) and you just roll rapidly left-right to about 20 degrees of bank, without doing anything with your feet (standard for PPL style flying nowadays), there's nothing to worry about.
However, if you want to do the perfect wing waggle, like the one used to denote the start of your aerobatics sequence (45 degrees, three times) you better have a big speed margin above the stall because you're going to lose some speed, and you're going to have to cross-control to keep on the heading. You might also lose some altitude. How to do things like this properly, taking into account rev limits on the engine (if fixed pitch), structural issues like Va and rolling gs, judging 45 degree lines and fuel management (with 45 degrees bank angle, cross controlled, you might not get any fuel to the engine anymore, depending on tank and inlet configuration) is clearly in the aerobatics realm.