The only person who can give you negative feedback is the seller, and they don't care when your bid comes in.
Since I buy quite a bit, I subscribe to software called "Bidnapper". I did it because MTPOCKETS4U, the zaniest e-bayer alive, told me he used bid software. It automatically puts in your best bid with a few seconds to go. I only do this for stuff I really want and put in my highest bid. Of course, you will only end up paying just a little over the next highest bid, so sometimes you pick up the item for a lot less than your max bid. It evens out over the long run.
To manually snipe, make sure you are signed in. Refresh the screen every few seconds, then send in your bid with 10 to 20 seconds to go. If the item is in demand, chances are you will get beat out by a bid program, but you never know.
Chris