It depends on the kit. Normally the first racking Primary to glass) you do not top up as there is still fermentation. When you stabilize and degass you top up for the clearing stage. It is acceptable to use up to a liter of water and most kit makers factor this into their concentration levels. If after that on additional racking you need to top off it is best to use a like type wine. It does not necessarily have to be the same varietal though. I have a batch of Wine Expert Vintner's Reserve Cabernet Sauvignon that was a bit thin for my tastes. I use it to top most red wines. I have a kit of Vintner's Reserve Italian Pinot Grigio that I use to top white kits if needed.
You will hear many that state they will never use water to top as they feel it will water their wine down but as I said, the manufactures plan ahead for this and increase their concentrations to allow this added water. Now of course, if you wish to top with wine every time it is fine but can get expensive. If you are good at racking you will have little head space. I myself and not real skilled at racking so I always play it safe if I am in my final racking and the wine is very clear that I don't suck up any sediment and top with a like wine.
One more thing I forgot to add. If you don't want to use water or a like wine, a lot of wine makers I know use sanitized marbles in the carboy to raise the head space level. It is a pretty nifty trick and you can reuse the marbles anytime you need. Just Clean them and then Sanitize them.
Smurfe beat me to it with the trick of the marbles.
Pebbles and marbles are great as long as they are utterly cleaned (boiling helps) and sanitised with sulphite so you are sure they will not bring any bacteria or off odeurs with them.
Another great trick you will learn in time is to collect all kinds of carboys and bottles in different sizes. So you just rack down to a smaller carboy. Trust me, in due time you will have collected a bunch of bottles in various sizes.