Never, Ever is a word that no one should use. It is a very strong word!
Anyway, who ever told you that must have a terrible experience with an open house. Thousands of open houses happen during the weekends. This is one way of showing your house to the market that sometimes leads to the sale of the house. As long as you put your valuables away and your realtor has the names and addresses signed in sheet at the open house, I don't think that there should be a problem.
Going to an open house is also encouraged. This gives the buyer and even the seller a comparison of what is out there and also learn about the market.