That's not safe spotting by definition (since there are no NPC guards). But it doesn't mean it's OK; it's certainly something that players would report and could be considered cheating. Despite it is perfectly within the normal mechanics of the game.
If you want to do that, then I suggest opening a GM ticket and asking if it is permissible. Make sure you screenshot the response if they say it is OK.
If you get some kind of vague response like "We are not banning players for fighting on roofs in AB at this time" then I recommend not doing it based on how bans are handled here. It needs to be a solid "that is permissible."
If you do that, let us know what the answer is. It sounds like fun.