How is it immoral? I relly don't understand that.
If the other bidder has put in a higher top bid than you then you don't win.
If the other bidder is willing to pay £25, but has put in a top bid of £20, and you put in £22 at the last minute then it's their fault for not putting in the top amount they are willing to pay.
That said I have never used a website to do it. I do often wait and watch an auction and then put in my bid at the last minute (or the last time that I am available to bid), but I find that helps me with my self-control, rather than putting in my 'top bid', seeing it bettered, and feeling that I have lost something and so wanting to increase my bid beyond what I was originally willing to pay.
There is a concept called 'loss aversion' that says that if you believe something is yours (for instance, if you have been told that you are the winning bidder) then you begin to value it more than when it was not yours. So your estimate of what it is worth gets increased. Bidding at the last minute (or alternatively putting in your highest bid and then not looking at the auction until the end) protects you from your brain telling you that you mustn't lose it and must increase your bids.