Dealing with the agent listing the home usually does not result in getting you the best deal on any property. So if you are currently under contract or obligation with an agent, I suggest you speak with them about visiting any Open House.
You could run into a conflict called Purcuring Cause if you like the home and make an offer through another agent. Any real estate agent would be able to explain this to you when they talk about Agency Relationships at your first meeting.
I would agree with the previous answer, you can schedule a private showing of most homes any day of the week.
If you still feel compelled to go to Open Houses, most Open Houses from all real estate companies are available on this web page.