All my husband's neckties (and yes... I looked, just for you) are "Dry Clean Only" or "Spot Clean Only." He doesn't clean his, but he also doesn't wear them to work so they aren't worn very often. He said that it's recommended to have them cleaned annually by a dry cleaner that has the proper equipment to clean neckties.
I verified this at this site:
http://www.neckties.com/content/styleguide.html
They say that regular dry cleaning processes can harm the color and shine of the necktie fabric. I know that this article isn't terribly helpful since you don't want to go to the cleaners, but it does explain how water is harmful to silk neckties. I also doubt that you'd get them properly re-shaped and pressed after soaking them in the washing machine.
Here's a user submitted article on cleaning NON silk neckties:
http://www.ehow.com/how_2198561_clean-a-necktie.html It sounds like it might work. I'd try it on a non-expensive tie first.
I also tried to find out whether home dry cleaning kits (like Dryel) would work on ties. No luck. I have a few clothes that are dry clean only that I use Dryel on between visits to the cleaners, but I still take the clothes to the actual cleaners every 2 to 3 wearings because they do a better job and they press them better than I can at home. I'm not sure how Dryel would do on neckties.
Good luck!