Hmm I guess it depends on how many bidders you have, if they are few or none I would just tell them.
I have done it asking for a reserve price on odd occassion, they probably just hate the bidding war and would prefer to just buy if they are interested BUT in most cases they may want to negotiate that reserve price
i would say depends on how badly you want to sell and how many people are bidding for it. I dont see the problem in telling them the reserve price, in my eyes its better for your auction really because if they bid reserve price then someone may go higher.
I really hate sending emails reminding people they have won please pay me whereas if they are communicating with you now they probably need what u are selling. JUST TELL THEM I say