Good to stop if you want to level in a zone and want to get all the quests done in the zone because even with time walking you end up leveling out of your zone way too soon and you begin to one shot everything including NPCs that you are supposed to injure and not kill for certain quests. Pretty much anything above level 30 is will be difficult to do any low level quests. 20 you can stop t at that and can go to TBC content and do e everything there. My level 30 toon sololed a bunch of TBC raids and dungeons so they are super overpowered right now even though 30 = 80 and 20= 70. A level 80 could not clear TBC raids as fast as a level 30 can now.
So for old world questing you wanna do that. For PvP if you want to twink your toon then you will go into a twink only bracket and will have a 4 hour queue so it will not be worth it at all.