After secondary fermentation, when I rack, add the stabilizers, and degas, I've been told it's best to top-off with additional wine (commercial or left-over from an old batch) to minimize the surface area exposed to air (basically topping up to the neck of the carboy). What considerations do I need to keep in mind? Will any commercial white/red work for a batch of white/red? For a concrete example, I'm doing a Sauvignon Blanc right now, and don't have any left-over white. Should I just go buy the cheapest Sauvignon Blanc I can find to top-off with?
It's true, you want to keep your carboys topped up. Oxygen is the enemy of wine, even moreso for white wines. Ideally you want to top up with more of your own wine, either from last year's vintage of the same wine or, better yet, from the extra you've been making all along just for topping up. But if you don't have any topping-up wine of your own, then you can absolutely use store-bought wine.
First, you should top it up with Sauvignon Blanc. You should not use the cheapest you can find, unless you want to lower the quality of your wine. You shouldn't use the most expensive you can find, either, because it could have a significant impact on your wine if the bottle you buy is different than your wine.
What you should do is seek out a bottle of Sauvignon Blanc with attributes similar to yours - similar color, similar body, similar nose, similar taste, similar acidity, similar alcohol level. What you are looking for is a topping-up wine that will blend into your wine such that you can't tell that you blended it into your wine. Wine blending to achieve the taste you want is an interesting topic, but topping-up isn't the time to do it.