I think what your grandmother means is that if a guy is rude enough to pee outside the toilet that means that he has no self control in terms of being polite in other people’s homes by making a mess in the bathroom(lol too much explanation) or literally not hygienic.
The real question is does that matter to you? Is this a dealbreaker? If yes, then your grandmother’s advice right if not then just ignore her advice. There’s so many aspects you can view this advice but ultimately you just have to ask yourself if this matters to you if your guy does that.
For example, I was raised on not making a mess in the car. I grew up not eating in the car or if I do I make sure that I will not make a mess. I was got use to maintaining a car clean. Now I am not naive. I have been in messy cars. Almost all my friends have messy cars especially during college. I was fine with it and it never bothered me until I started seeing a guy who likes messy/dirty cars literally all his crap have to be in the car and actually prefer it that way. He just grew up that way and honestly that’s ok. But obviously it was a dealbreaker for the both of us. TLDR He can’t stand a clean car and I can’t stand to have my car become messy.
It’s all just preference so really it’s up to you.