How terribly important is it to worry so much about a little air in a keg when you're filling it? nd won't that little air in the 1" of head space in a bottle get used up by the yeast?
Probably right, at least about the bottles if you're bottle conditioning. I'm always careful to purge bottles however because I only
bottle bright and already conditioned beer, usually quite a few months after most of the yeast has surely dropped out.
Still, I wouldn't think of transferring even into the keg without purging it out first since it's absolutely no trouble to do so. It literally takes little more than a minute. That way, I also minimize any worry about my kegs full of barleywine or 18th century porter sitting at cellar temperature for more than half a year.
Necessary? Who knows. But it's a quick, cheap insurance policy.
But, of course, everyone has their own individual procedures. I've been following mine for 2 decades, so while I've tried and adopted a number of new things since I've been a member of the boards, I'm pretty set in my ways as far as the co2 purge goes. It takes virtually no time, does no harm, and probably does some good...so why not!