I always use 98 ron just for the peace of mind, and in my opinion the car runs a lot smoother on 98 than premium (95). Once I added octane boost to 98ron fuel and it ran smoother again. They are tuned for 100+ron fuel so you might as well get as close to that as you can. Sure its probably not necessary, but why skimp? Asuming you save 5c per litre by getting premium instead of 98, this is only a saving of $3 per tank - which is hardly significant.
Cheers
Greg