I guess I should share my cautionary tale....
I have been a host for a few years, and only once did it go really poorly. Two years ago I was asked at the last minute to host someone. I was a "regular" at interview days, typically doing some of the grad student interviews and handling a Q&A, so it wasn't a big change to my schedule....or so I thought.
My person was pushy, rude, and a complete mess with her scheduling. She missed her flight from ATL (stuff happens), so she informed me she was making her bf drive her down, and that they'd get in around 11pm. She never asked if I'd mind they coming in really late, that her b/f was coming, etc.
She arrived around 1:36am....I remember because my guard called me (gated community), and I had just gotten to bed about an hour before. She didn't apologize or anything, and pretty much just brought their stuff into the spare bedroom and went to bed. The next morning they got up, and she didn't say more than 4 things to me. The b/f ended up being a really nice guy, and he apologized for coming in late, thanked me for letting him stay, etc.
After leaving my bathroom a mess, the spare bedroom a mess, and barely saying, "hello"....I wasn't in a good mood about the whole experience. While she wasn't one of the applicants I interviewed that day, I left a note in her file summarizing my impressions of her, her ability to "fit" into our program, and my recommendation. I can't say if my note made a difference, but she was not offered a spot in the program.
Even if you aren't officially being evaluated......you are still liable for your behavior.