Make a couple of reservations. When you get to the counters pick the one with the shortest line. If you are traveling with someone, each take line and see who gets there first. if one which you don’t have a ressie has no line, ask them if they have a walk up rate, if it beats the others or you just want to be out of there, take it.
Whenever you have ressies always ask for the walk up rate before you say you have a ressie. If it is less than your ressie, take the lower rate.
After you leave the counter, cancel your unused ressies.
One time I did this, my ressie line had a few people and the next one had no line. I saved $50 on a 2 day rental and a long time in the line which had barely moved since I got there.