Ever since I moved my sims to their villa up in the hills of Sunset Valley, I built a wall around the lot, and in the front of it there are 2 gates, being one of them locked to everyone but my household, and the other one is the electric gate, so that when a guest/maid/whoever comes over they may still come in. I also lock all bedrooms (except for the guest bedroom), private bathrooms and band studio. Even if a guest ever gets allowed inside, I still don't want them to mess with my sims' instruments or personal stuff.