Oxygen is wine's worst nightmare. If you top up the carboy to the neck there is less oxygen to wine surface contact, thus less likely to oxidize or something along those lines. If you rack into secondary before the wine ferments dry it will continue to produce co2, which should occupy that space anyway. The problem you may run into is after racking and the wine ferments to dryness, then you rack it again. You will lose some volume to lees and what-not and then there will be a large pocket of air in with your wine, this can be a problem if you need to age it for six months or more. In a pinch I would try to rack the wine before it's done fermenting, to fill the space with co2, then rack it once it's nearly done and then immediately degas by pumping the co2 out of the wine.