I've never heard "trim back" before, but I'd recommend every 2-3 times your brew. Personally, I clean my SCOBY and my vessel every single time. Pour some kt into a clean sanitized pot, wash your hands in it thoroughly for about 15 seconds, put your SCOBY in there, peel off the top layer, dump the rest, and rub away any dead yeast colonies (the dark stringy stuff) before putting it into the next batch.
Tap: DIPA, oaked Baltic porter, XXX pale ale, dark IPA
Bottle: vanilla kombucha, oaked puh-er kombucha, blueberry kombucha,
Primary: rose hip kombucha, oaked honey and oolong kombucha, hopped kombucha brewed with green tea infused with cab. grapes