I'd be curious what percentage accept/reject on the spot, but I'd never ask a candidate to provide an answer either way if you present an offer right away. It's too important a decision for them not to at least take the night (especially after a day of interviews, a flight/hotel stay, an impending flight home). Even suggesting an immediate answer feels salesy ('whaddya think?'), and I'd think we'd see better results by presenting the offer and saying "take this home with you and get back to us when you're ready".

This approach should also tend to reduce counteroffer acceptances or accept/renege scenarios.