well, my first tape (slint @ 930 club) i did 18/30 the entire show. results were a bass-heavy recording.. some brickwalling.. but this was a very loud show with BOOMING bass... my mics couldnt handle it. maybe if i had set them lower (around 12/13) it would have turned out better.
2nd tape, the club (warehouse nextdoor) is very small (maybe 125 max capacity, and it was pretty packed) and the bass is not nearly as loud. i was about 8 ft from the stage DFC for most of the tape. i set my levels at 16 or 17/30 i believe (i think 17).. anyways, tape came out great... NO brickwalling.. volume was a tad low, but i gave myself enough headroom to raise the volume a few db in post, without clipping.
so i guess it all depends on the venue.. i'd probably be too scared to set my levels above 20