I have seen some units with a pressure switch on fresh air side which doesn't allow the exhaust side to run unless it is. But I've also seen them without. I'd imagine that is very model/manufacturer specific. Add that feature to your checklist when you're doing your research to buy one.
If you have a gas fired appliance like a hot water heater and/or furnace, those particular rooms really should have combustion air provided to them already (unless they are high efficient direct vents with plumbed combustion air to them).
If yours doesn't I'd be tempted to have someone take a look at your situation to see if you need it. Many of the older homes have so much leakage, it wasn't a concern. But as people build or renovate their homes and seal things up, it becomes a greater concern.
Keep in mind too, what all these things are actually doing. There are instances like was mentioned earlier where my conditions are totally different from some one else who lives on the other end of the country. It may be that HRV's or ERV's are simply not the answer. In warm moist climates these units may actually do more harm than good. If the air outside is warmer and moister than inside - ventilating all you want won't help. In these cases a stand alone de-humidifier may be your answer. Here's a great read on some things to consider as well.
http://www.greenbuildingadvisor.com/blogs/dept/musings/hrv-or-erv