Yes, I have raised my snipe amount when I bid on an important item and had given it a little more research or thought. I also have manually bid on an auction a few seconds before the snipe is placed in such auctions, just for assurance. I know that it raised the amount I owend the buyer by $1.00 on the last bid/snipe I did for around 1800.00, so it was a cheap safety-net, I think.
As for bidding early, I also vote never!