I believe if WMP has been started, it works. That includes even if you later exit it. If I reboot and do not manually start WMP, then the PC doesn't appear on the PlayTo list on other computers on the network.
I also have a (probably) unrelated problem which I am amazed when I search on it doesn't return any results. That is that songs very oftwen play 2x. I have repeat off on the PlayTo Source machine. I have no idea why it is doing that. It appears in the play list on source and target machines only once!