No. I've actually beheaded a player for this, then set fire to his body in his own living room, in front of his wife and children.
Okay, so I didn't. Seriously, I voted yes. I'm fine with this. The way I see it, as a DM, I'm plotting against the players all the time, so why not let them plot against me.
I voted yes, because I've never had any reason to disallow it, but I don't use it frequently, and I agree with the inspiring poster that it can be part of a bad vibe. Some granularity to the poll might have been a good idea.
Yes, but too much of it can be annoying. Like too much going out into the hall for secret conspiring. A little adds a bit of spice, surprise and suspense. But a lot could detract from the player co-operation that's essential in my view. I see D&D as basically a team game.