I am in favour of clean ass.
Initially, I was pretty ignorant in that matter. All my experiences with anal were limited to anal sex with females, and aside occasional spot here and there, it had never been any bigger issue. Just being careful when removing condom. I though that a bowel movement was sufficient to address potential issues.
Then I got involved in BDSM, as a submissive, and found myself on receiving end of anal play. At the very second meeting my domme scolded me I was not clean inside. That was followed by my very first enema ever. There was a bit of dirt on a toy, but I think it was just an excuse for the enema.
Nevertheless, since then I am religious about being well cleaned out before play.
And as @tallslenderguy mentioned, thanks to porn, AtM became a thing that is almost given with Mm or MMm play...
When I am in dominant role, I tell my sub that I expect them clean or I will give them enemas. So far all elected to do the clean out themselves.