We picked 17 for Elliot falls mainly for close to facilities but far enough away plus there was a tap about 20m away from us. we were there towards the end of August and had 2 other campers in the whole site.
Main thing to watch for is if you are running early it can be difficult to notify the parks we tried had not service and did the detour out to ranger station only to find the gate locked and not going to walk 2k's in the heat to get to the station. Decided to just go and camp and if the ranger came out just see what happens,(never saw him). After talking to some others at canal creek they said don't worry about booking as that time of year is very quiet and the rush is over.
Dave