I don't know where you're getting that from. You can indeed leave all the registers alone and it will repeat whatever is there for any number of scanlines until you do change it. In fact, this is what happens if you power up the 2600 without a cartridge - you see vertical stripes, because whatever garbage was in the registers on powerup just stays there and repeats onto every scanline, with no software changing it.

"Sometimes forward and sometimes backward" would only apply to the three registers of playfield bits, and is fully controllable by software, in the CTRLPF register.