Guild Wars 2

Should me and a group of friends want to do some PvP unbothered, is it acceptable to go into someone else un-used PvP server and play there? I would do it, but I dont know if its some Taboo X.X

Yes if it doesnt have a password, but if they do come to their arena dont get in their way. Back when my guild had a Custom Server we didnt mind who was there as long as they didnt get in the way of what we were doing.
(We had a pass but didnt mind giving it out, but the fact that you got to pay to upkeep a custom server is stupid.)