im not sure what the experts might say but for me i pick the one that sounds the best. on a les paul the rhythm setteing is usually the neck pickup and gives a softer more balanced sound while the treble setting is the bridge pickup and gives a chunkier harsher sound.

if your guitar is setup right the neck pickup should be farther away from the strings than the bridge pickup and it should be the same on any guitar. this combined with the distance between the pickups is what makes the different settings of the toogle sound different.

the best thing to try is to find what sounds best for what you are playing.