Purging air from a pressure cooker Many years ago, while instructing me in the use of our pressure cooker, my father told me to always let steam escape from the top stack for a couple of minutes before putting the weighted cap on it. This, he said, would purge air from the cooker and thereby prevent oxygen from attacking the food at the elevated pressure and temperature inside. 

This makes a lot of sense to me, but I never see it mentioned in pressure cooking books or instructions. What's the deal?
In pressure canning, the USDA says:


  Air trapped in a pressure canner lowers the temperature obtained for a given pressure (for example, 10 or 15 pounds pressure) and results in underprocessing. To be safe, USDA recommends that all pressure canners must be vented 10 minutes before they are pressurized. 


Which might be where your dad borrowed that idea from. The effect in that case is more from the air taking up space that should be filled with steam, and preventing it from being filled with steam (rather, being filled with a steam/air mixture), making the transfer of heat from steam to jars less effective.