First off, you may be asking, what is sniping in regards to Ebay? A quick search yields no information on the subject, so here goes.
Sniping, simply put, is when a buyer waits until the last possible moment to enter his bid into an auction. What is the use of this, you may ask? Well, it's mainly used to discourage emotional overbidding, which is where someone goes over their initial bid, after being outbid. Supposedly this is out of being upset or annoyed, but it could very well be that the buyer was comfortable with his former, lower bid amount. In either event, sniping pretty much ensures that by the time the outbid notice winds up in the snipe victim's e-mail, the auction is over. This can be frustrating, and some auction services actually go so far as to extend the auction time additional hours if someone places a bid last-minute, but Ebay doesn't do this.
So, generally, it's almost always a good idea to snipe if possible, rather than place your bid early in the auction. But there are exceptions to the rule.
Exceptions such as Buy It Now, for example. This little monstrosity Ebay dreamed up allows sellers to set an ideal price point, in addition to the initial starting bid price. Anyone (that goes through the verification process) can simply elect to pay that amount and immediately end the auction. So, what's a sniper to do? Well, interestingly enough, any regular bid (even if it's under the Buy It Now amount!) on a Buy It Now auction will disallow Buy It Now from being invoked. So, if there are no bids, plunk down the mininum bid, and snipe towards the end as nessecary.
Really though, all the other ideas involving when not to snipe are theoretical. One could say that placing the initial bid on an auction can discourage further bidding early on, but on the same token, some people are drawn to auctions that have been bid on, out of sheer curiosity.
Really, the final point is that sniping is hardly the exception, but it's certainly not the rule.