Black Diamond gear pimp Roger Strong, fresh off his profile in Rock and Ice Magazine, has a nice little video demo illustrating the correct placement of an ice screw. He mentions the 10 to 15 degree upward angle placement, which is always worth saying.
Just last season my partner and I watched a guide explaining to her client on the adjacent climb how it was important to place the screw downward. There's even a comment after Roger's video about how if there is melt-out danger you should place it downward.
Bad idea. Always place upwards. If you're worried about melt-out, find another placement, cover it with snow and/or climb faster to get to your next placements. Relying on leverage to protect a fall on a screw is just bad advice anytime. Know your pro!