I always, but always, top up meads and wines. If there is too much headspace, I rack to a smaller carboy. I have several different sizes, and if I have, say, 5 gallons and a little bit, I rack to a 5 gallon carboy and put the remainder in a sanitized wine bottle and use it for topping up. I always plan on having a little "extra" in a wine bottle or grolsch bottle for topping up. If you don't, you will have a problem with oxidation. After a racking or two or three (or more), very little co2 (if any) is being produced and I even use campden tablets in every other racking to ensure that my wine or mead isn't at a risk for oxidation.
Good meads and wines can sit in a carboy for months- that's another reason to minimize the headspace. For a week or so, it wouldn't be too much of a worry. If you've ever had oxidized mead (I've had one wine that became oxidized a little because of a poor fitting carboy cap), you'll understand. It's an unpleasant, sherry-like taste and it actually can turn the mead a tan/brown color. It will ruin the entire carboy full of mead- and a batch is way too expensive to risk!
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