To answer the OP
S-tronic can/will creep
S-tronic can also roll-back. (Which is why you have around 2 seconds of Hill Hold as standard. Hold Assist is an option, but Hill Hold is standard on all A3/S3 Manual and S-tronic)
In "S" - with engine running, and you're in "D" - the clutch will be held at bite point with your foot on the foot brake. If you're going to be stationary longer than say 40 seconds, nudge the lever into "N". It will save the clutch being constantly at bite point.
In "D" - with engine running, and you're in "D" - and you have your foot "lightly" on the brake - this is the first brake pedal position where the car is stopped just enough to stop it creeping, and S/S hasn't cut in, in this situation, the clutch is at bite point as above, so again, if you're going to be stationary longer than say 40 seconds, nudge the lever into "N". Or Press the Brake Pedal harder, as this will lead to:-
In "D" - with engine running, and you're in "D" - and you have your foot "hard" on the brake - this is the second brake pedal position where the car is stopped, and S/S will cut in. In which case nothing is turning, and there will be no clutch wear in this situation. If S/S is disabled, then engine will continue to run, but revs will drop, clutch plates back off a fraction, and there will be less clutch engagement compared to the scenario above. In theory, you can stay in this situation as there is minimal clutch engagement (but still some), so it's up to you and your degree of mechanical sympathy if you want to nudge the lever into "N" or not. I personally would if I was going to be longer than a minute. Plus it saves those nasty bright brake lights
People with S-tronic AND Hold Assist can find a 3rd brake pedal position, where S/S engages, but HA doesn't
Or some such combination..
At the end of the day, it's going to take many many miles to wear out the clutches by holding them at bite point at idle, so one could in theory never use "N".
However it is also true that there is clutch engagement at all times due to the nature of the gearbox, so it won't hurt to have a degree of mechanical sympathy and nudge it into "N" for longer waiting times.