I'm usually a spectator until just before something closes.  The earlier you bid the more time you give other bidders(or seller) to notice and react.  The only large item I've even won I only bid one time with a few minutes left and got it.  It's exciting to be bidding that hour before something closes thinking "what if I get it for that?" but I never do steal anything.

Edit:  I now realize you're talking about actual in person bidding.  My comments were referring to online bidding.