My printrbot idler sits too low. The belt does not run paralell to the rods so the tension changes as the carriage travels. I did print a modified idler bracket but I've never fitted it. I don't really like the belt teeth riding on the skate bearing and it turns out adding an 18T adaptor improved the belt angle. Maybe it was planned that way?
I have a feeling this has been done before. Anyway here is a sketchup version. Credit to droftarts who did the hard part.
My X - carriage seems to be running smoother than before.

If your belt is already running true then adding this may be a bad idea.