It depends on how much interest there is in an auction as to wether last minute bidding will help or not. When I bought my Mirage OM-10s off Ebay, there was me watching, and one other guy bidding. Strangely nobody else had any interest, so I waited until the last 10 seconds or so, outbid him by $5, and got the '10s for myself at an absolute steal, $350 for the pair.
On the other hand, if there's 10 or 15 people in a bidding war in the last few hours of an auction, last minute bidding is basically worthless as the price will have already shot up by huge amounts.