Keep in mind that an Agent at an Open House most likely works for the seller, just as an Agent at a model home most likely works for the builder. You should have your own Agent, representing YOU.
Fonville Morisey Realty
Be up front....contact the agent and tell them you have interests in the home but feel you need personal representation and will be having an agent contact them to see the home a second time with them.
Your concern is a very real concern. If you plan on aggressive negotiations, you efforts would be best served by an agent that is a step removed from the sellers.