Although I'm sometimes frustrated as to why my auctions don't sell for the same price as others up at the same time (like why there seem to be 2 or 3 folks willing to pay $200 for a print that I can't even get a bid at $100 for), I haven't and don't think I could get to the point where a shill bid would be justified. If you aren't willing to sell at a price, set a reserve. If you don't want to pay the reserve price, you probably shouldn't be putting stuff up for auction IMHO - just try and sell thru other routes or lower your asking price.
Again, just my opinion. But ethically, I just can't do it. And I do have a hard time bidding when I feel that my price is just being driven up because someone wants to get a fair price for their print.