I must be the odd man out, most ponies that come my way get cleaned inside and out. I live in a humid area and I have not yet had the funds for a dehumidifier for my pony room. Where possible I remove the washers from ponies while others that I know don't have heads that come off easily, I leave them on. If the glue pops off easily though, they're getting cleaned. In several instances I've been able to nip serious problems in the bud before they ruined the pony. Outwardly the pony would look fine but then I opened her up and -wham- the beginnings of mold.
So yeah, I most often remove their heads and leave them loose. My ponies are staying with me so I'm not worried about their resale, I'm worried about preserving them.