I would probably discourage the type of dobbing that is done to simply get another party into trouble and try to teach them to work out their issues constructively. But I don't discourage dobbing when it gives me information that stops someone from getting hurt, or helps me out.

My son so far doesn't dob to get his sisters in trouble. He's only 3. His dobbing is usually:
"Muuuuuummmm, Shayla is standing on the table" (don't stress, it's just a little low table in the play area )
"Muuuuuummmm, Arlia's done spitty banani" (that means she's vomited)
"Muuuuuummmm, Shayla's throwing her food"
"Muuuuuummmm, my socks are falling down" (dobbing on his socks hehehe)

So whilst some of them ARE dobbing on his sisters, his intent is not to get them in trouble and he's actually doing it to try to help me out.