This depends on what sort of server you're playing on. If you're on a PvP server, avoid wandering into other players' bases to use their map room unless you have their permission. However, if you're playing on PvE or PvE-C where your base cannot be destroyed, and PvP is either disabled or time-restricted, the owner of the map room has likely chosen to leave it out in the open so that people can use it. As long as you're not having to jump through their defenses and clamber their walls, it's probably a safe bet that you can use their map room, so you could use theirs. However, bear in mind that they may one day move the map room or stop playing on that server, so a time may come when you have to craft your own.