Depends on the question. If the tag would obviously be relevant and enhance the search-friendliness of the question it may be appropriate to add such a tag. For example, once I saw a question about how to add a GUI to an existing C program. Tcl/Tk was an obvious fit to their problem from the way they described it and I answered describing how one might use Tcl/Tk to do this (Tck/Tk was originally designed to do this sort of thing).
As the original tagging on the question wasn't very good, so I retagged the question anyway and added a Tcl/Tk tag. Whether the tcl-tk tag was appripriate is something of a matter of opinion, bit I believe that it was entirely on-topic for the question.
I think this is quite a good example of a corner case where you could argue for or against the tag. I made a decision and implemented it and it probably did more good than harm to do it the way I did. Someone else could have decided differently. I don't think there's really any hard rule either way.
In your case with the WCF tag, if you didn't want to use a WCF based solution you could have clarified the question and commented on answers that suggested WCF. As the owner of the question you could have removed the tag. In the case where a tag is definitely not appropriate the owner or anyone with sufficient rep can re-tag the questoin.
So, I think it's not appropriate to make blanket rules either banning or mandating this practice. There are too many possible corner cases and the tagging is not hard and fast - it can be changed later and might signal that you need to qualify a question with (for example) a a note with a reason why a particular technology is not acceptable.