Thanks Roni; I thought it probably was not a good idea (but didn't really know why not), and your explanation explains why.

My dog reacts protectively when strangers come near the house. Usually all she will do is one or two low growls when she hears them, then looks at me. I have never corrected her for the growl, but I re-direct the same way others have already described. Usually, I do the "out" and have her sit. If it's someone coming over (friend she hasn't met yet, etc) I will have her meet them (quietly and calmly) after she has seen my reaction to them.
There have been a few times where I have not given the "out" and have simply told her to sit. I haven't praised or encouraged the growling, but I haven't told her to stop either. The difference in my reaction to the person kept her wary, but she was still following my lead, not deciding for herself what needed to be done. In all but one case, the person was lost, or wanting to admire my dog ;-). In those cases, I would give her the "out" and have her meet them. In the one and only case that I didn't eventually "out" her, I waited until the person was out of sight,(she never took her eyes off the guy) then did a recall to my dog and praised her for a great recall.