Personally, I would not leave them open. I have always used a blow out adapter (Camping world http://www.campingworld.com/shopping/item/blow-out-hose/69232) that attaches to your water intake and allows you to run air through your lines. Just be careful how much pressure you use. I would set my compressor to a max of about 50psi. Open each faucet individually. You will be amazed at how much water comes out. Don't forget to drain the hot water heater. I usually achieve this by removing the anode rod (if so equipped) and opening a hot faucet inside to assist the speed of draining. Replace the anode rod. Then you add your RV antifreeze. Hope this helps.