I have only ever used 95, usually Tesco in my S3. Not had a problem.
My understanding is that to extract the maximum power the engine management will automatically advance and monitor the timing until just before the engine 'pinks' or pre-ignites. The extra timing advance that running 99 Ron will give about 5% more extra power than 95 RON.
Back in the day, before this feature (introduced around 1988), many engines had a reversible plug to manually tell the ECU if you were running 95 or 98 RON. Also many high performance engines had, like my 245 BHP Calibra Turbo, to have the ECU modded to run only at 99 RON in order to achieve the power.