I flush my mash tun valve.
I recirculate hot oxyclean through my pump and boil kettle valves, then rinse, then starsan.
Normally disassemble and clean ever 4-6 brews. But I'm replacing my ball valves with 3 peice ball valves, which will be cleaned every brew day. The gunk I find when I take apart the old ones grosses me out. Never had a problem, and the beers made right after thorough cleaning are no different, but... freaks me out and probably a matter of time.