Some people put cheato, some put LR, some nothing at all. If you do put LR back there just be aware that in a short time lots of detritus will deposit on the bottom, so you should clean out the chamber every so often.
Using AC is a personal choice. It does have its benefits (assuming its good quality and doesn't leach Phosphates into the tank) such as removing yellowing substances from the water, but needs to be changed out every so often since the pores will clog and become ineffective.
FYI - If you start cleaning too much or too vigorously you will likely see algae and/or dino blooms in your tank. Start slowly and only vacuum a small portion of the SB each week. For my 4 year old tank I have found that a SB of 1" or so works best, but that's not to say that a 3" SB can't work too if it is well cared for.
Good luck.