If it's a car I'm familiar with, I'll tweak sets that I've already been given. If I see someone doing amazing times, I'll normally ask to try it, but I tend to go back to my own as most of the fast sets are horrible to drive.
For example, I only use around 5 or 6 different FOX sets, and if I REALLY need another one I'll tweak one of the others.
If it's a car I don't know, I'll ask for a set of someone online who's doing quick, consistent laps. If I see someone I recognise online whose sets I normally like I'll ask them first.