Fermentation should not be occuring in the secondary. After the first 7-10 days in the primary, your beer has pretty much completed fermentation. You typically want to avoid headspace in the secondary at all costs. But that is not to say that some headspace in the secondary (without flooding it with C02) will ALWAYS ruin your beer. It depends on how much headspace you're talking about, if that space is being taken up by a bunch of leaf dryhops, how carefully you racked, etc. To limit oxygenation, I would recommend that you only rack to secondary when needed. And if you do, be sure to fill it up with as much beer as you can, leaving as little headspace possible.