We have sold many homes and I have toured more houses than I care to remember. I can tell you from experience that yes, a number of the houses DID have signs up asking visitors politely to take their shoes off. Usually there would be a mat outside for guests to place their shoes on upon entering.
Most people don't take their shoes off automatically when entering another's house. Please don't think they're trying to be rude, as it's a cultural thing. It really is not done as much as you might think except in homes that tend to be more formal. Most people are more casual these days.
However, if you would like to request that they take their shoes off, you can tell your realtor to note that in their information. All realtors keep specific details noted on the paper with the house's specs. In addition to the square footage, number of bathrooms and bedrooms, etc. they will also have other info. This includes notations like if you have a dog, if the dog is friendly or shouldn't be approached, if pets are allowed inside the house, if visitors should lock gates behind them, what the code is for the lockbox, if people are required to take their shoes off, etc.
Don't feel guilty or bad about requesting this from other people. Like you said, it IS still your house. Not only that, but it's a smart thing to do because having lots of people come through your house does bring in a lot of dirt. Ignore their bad attitudes, or gracefully chalk it up to poor breeding. Just put up a sign on your front door, and call the realtor to tell him or her you want people to take their shoes off.
edit: And by the way, you don't have to worry about explaining yourself to other people. Asking this as a courtesy does not need an explanation. And you don't need to go out and buy little booties for them or plastic strips for the whole house. A number of homes for sale do request people take their shoes off. I have never noticed anyone refuse to do this.