If you purged the keg properly, I'd say it doesn't matter how you go in, there's no O2 to pick up. If the keg wasn't effectively purged, also doesn't matter how you go in, you'll pick it up. Sounds like you're good if you purged with sanitizer pushed out with CO2. I do go in through the out post, (wait, which album is that...?) and like BrewBama, I like the gas to be pushed out the in post. When beer starts to flow out the gas post, the keg is totally full. (EDIT Sure you can keep pulling the PRV and wait for beer to come out there, but that's messy, and I'd worry unsanitary. ) Then push out a pint or so then usual way to make headspace, and you know you have a keg full of nothing but beer and CO2.