I can't remember who it was, but one demonstrator suggested that unless there's excessive rust he tends not to lubricate the bed and do much maintenance on it. The thinking is that the banjo will lock better when there's a slightly rough surface for it to sit against. Kind of makes sense as long as it's easy enough to move it around on the bed, which I guess depends on how much space the locking mechanism has.
I tend to lightly sand and lubricate after wet turning but that's about it - I've experienced the banjo moving when not wanted on an overly smooth and waxed bed.