But one thing I’ve noticed playing tabletop (with 1 square to 1 inch enlargements), is that initial positions of monsters are generally not worth erasing, since players spot them immediately anyway. Secret doors and things that need to stay secret are a different matter, of course.

As a DM, when situations like that come up I usually say something like “Interesting!” and then either come up with some right there or tell them I’ll return to their idea once I’ve thought of how to handle it. Then I usually see what other people are up to. If the request is something really out there or unanticipated, I might ask the PCs to plan amongst themselves while I throw together a quick encounter or something like that. Generally, any dead time in my games is filled with in game planning or out of game joking around, so in either scenario it’s not totally out there to spend a few minutes thinking of something.

So generally I’d say: take the time you need to come up with a response and don’t be worried about the players seeing through your illusion; they’re most likely to just get a kick out of coming up with something unexpected anyway, especially if something cool and fun comes out of it.

In the forums. There is a 4e DnD edition that has been getting regular updates. I`ll be doing a full review of it here soon. VERY nice mod for 4e DnD.