You know, I feel like I've seen this trope a few times, and it's enough for me to know that I don't like girls who'd nonchalantly bathe at the MC's house. Sure, on one hand it can be said that she trusts him enough that he won't attack her while she's at her most vulnerable, but on the other hand it's such an out of place thing that shouldn't happen until they either start going out or are living together, married or otherwise. It's also a subtle hint at showing how much of a doormat MC is, and with how much he had put up with in his previous guild, it kind of matches. First it's just barging into his house to use his bath without permission, then the next thing you know she'll be asking him to get her favorite soaps and hair products like she owns the place, and soon enough we'll be back at square one where the story began.