It really varies. If everything else is okay, that's not something I would pick on. Small things are okay. Having them come back may be better than saying something. I mostly keep to public messages and try to phrase it so that it could be read publicly without violating any policies, but only if I think the guest really deserves it. This is more meant to warn other hosts if I see an issue. The only time I used the private part was to let him know that his friends peed in some cups and left them in my closet.