On my first better bottle, I did as mentioned before and used an L-shaped carboy brush to clean. That was a bit over a year ago. After cleaning that way through 8-10 batches, I got anal and heard about scratching plastic. I do notice now some circular scratches in that BB that show themselves at certain condensation levels. Since then I've bought 3 more BB's, and I made the rule never to touch the inside with anything but liquid.
By the way though, I still use that first BB (with the visible scratching) when I am out of other carboys, and I have NOT developed an infection in it. When cleaning it now, the scratches hold onto gunk longer than the clean parts of it do... so I'd have to tentatively recommend against using a brush. No biggie though.
Those are my personal experiences,
Cheers