The local LHBS's beer kits make 6 gallon batches, so I always make that format. I ferment in a 30L (~8 gallons) bucket, and then rack into a 6 gallon carboy. This works very well for me, as I don't have to worry about jammed up airlocks etc, and it leaves relatively little headspace in the carboy once racked.
For my first partial mash, I took a recipe I found here which was for a 5 gallon batch. I decided to ferment it directly in my 6 gallon carboy. I reasoned the good amount of headspace would leave enough room for the krausen (which it did), and the space would eventually be filled with CO2.
I know it's not strictly necessary, but I do like to secondary my beers.
My question then: if I rack this batch, I'll have "1 gallon" of headspace filled with air. Should I be worried?