I don't actually think it's possible to use "stealth" in PnP the same way you do in NWN. As far as I know, you need some kind of cover or concealment to hide at all, and then anyone whos line of sight you enter gets a spot check to see you. And you can't hide while your in anyones field of vision at all. That's what HiPS is for. However, you could use a bluff check in order to attempt to distract someone so you can hide.
As far as in PnP, I believe HiPs just allows a Ranger level 17th or higher to use the Hide skill even if someone is looking at them. But still, it would be hide vs a spot check.
But as far as when to advise someone on PvP, or when not to do so OOCly. I think it's a case by case basis. If your an Assassian, and someone has hired you to take out someone, as long as that was done purely IC. I don't see why you should warn them, more over, just make sure the person asking you to do this has a IC reason.
However, I do believe if your a monsterious character and you plan on attacking, or killing someone, some sort of RP, or OOC info on what's happening is usually nice. And everytime I have encounted this situation I have always felt like I was well informed, and really enjoyed the RP.