I try to take a different bike each time, but I don't have any sort of rotation. My choice might be influenced by what the ride conditions are or I'll just grab one that I haven't taken in a while. Sometimes one will get more attention for a while if I have made changes to it. I use a Garmin, so I can periodically check the activity record to see if one is getting neglected. I don't worry about tracking the maintenance, but I feel I have reached the point where it could be a chore to keep them all current.