Lucijay, you will have no problem, as it is impossible to bid against oneself on eBay. If you bid $20 for an item that someone else has a maximum bid of $10 for, your bid does not immediately go up to $20; it stays at $10.50 until someone else bids higher. Then your bid goes up to the next increment necessary to be the highest bidder, until someone outbids you.
So in the example, your bid shows at $10.50, even though your reserve amount is $20. When someone else comes along and bids $15, your bid amount then shows as $15.50, which is one increment above the other bidder's $15 bid. You are still the top bidder, until someone else bids $20.50 or higher, in which case he takes the high bid.
Even if you go back and put in a higher bid (say $25), if no one else bids above the $15 guy, then you still take the auction for $15.50, even though you put in a higher bid "against yourself." Ebay's proxy bid system only uses the amount necessary to win, regardless of how many bids you put in.
In the case of Auction Sniper, they put the bid in for you, but it still shows up as YOUR bid. Ebay has no way of knowing that your bid was placed by AS, so the same rules described above still apply. A last minute snipe may be higher than your original bid (the one designed to get rid of Buy It Now), but eBay will still only use the amount necessary for you to win; you cannot bid against yourself.
The situation you describe is the only time that most of us AS snipers put in our own manual bid early in the auction: to get rid of a very high Buy It Now price. (If the BIN price is remotely reasonable, I usually just buy it then.) Otherwise we lie in wait until the final seconds!