To just blindly answer to the topic;
Yes, what could be more natural than pissing when you need to?
As for if it's "accepted" in our time, that's another thing.
A few years ago, a law was made - here in Helsinki, Finland - that made peeing outside (/in public places) illegal. They even mobilized a group of people guarding this law.
EDIT: Sub posted while I wrote, yeah, I pee when I need to - no matter where I am. Of course I try and hide, but I do pee outside now and then.