I sold my condo in 3 days, so I never had to deal with it. But from what I understand, an open house isn't so much to get a buyer as it is for all the local agents to see your home, so if later they have a client that fits your house, they can say "aha, I've got the perfect place for you". I guess that getting a sale from an open house isn't terribly common (but not unheard of). Ask your agent (if you have one) and maybe they could do an agent's only open house instead. Your agent gets a group of other agents and takes them through the house, hoping they have a client that would be interested in the home.
If house sales in your area are strong, I'd put off an open house for the first, oh say two weeks it's on the market. If it's showing well and/or you get offers, you might not need one at all. If showings are weak, then you can re-consider an open house.