Is there any reason to keep original drive-train components together until they can all be replaced? I broke the master link in my chain a while ago and my LSB told me it was because I wasn't cleaning/lubricating my chain often enough. The chain had experienced more wear than it should have as a result. I asked about replacing the chain and was told that you typically don't want to replace individual parts of the drive-train, that mixing and matching new chain and old chain-rings is a bad idea. Is there any truth to this? Why would a new chain on an old (we're talking less than 4 months) be a bad thing?
Breaking a master link, as already said, is not so common, and is usually not related to the chain wear. It can be related to loss of lubrication, for example when the chain RUSTS and gets some hard links that cause the chain to get entangled with the derailer while pedaling, for example. But only because it is worn, supposing it is clean and lubed, I'd say no.
Installing a new chain with an used cassette might be divided in two kinds of situations:

You install and the chain doesn't skip. This is the ideal situation, meaning you replaced the chain at the right time, and your cassette will last much longer (note: this is the recommended thing to do - use lots of chains with the same cassette, replacing them often as soon as they get some wearing);
You install the chain AND IT SKIPS over the cogs when you pedal. That means you waited too much to replace the chain. It might be very difficult or almost impossible to use the new chain and make the cassete wear "back to normal". In this case, it MIGH BE a better idea to keep riding with that chain for a bit more and replace it together with the cassette later (waiting too much can be dangerous, since the chainrings might get damaged in the process). Even when the chain skips, though, if it skips only while pedaling hard, and/or only on specific gears, it is usually possible to ride slowly in the beginning, and the skipping starts to disappear as the cogs wear, and everything might come back to normal.



So, the shop guy may or may not have told you the truth, depending on how much your cassette is worn. This is not an easy thing to do if he didn't measure the chain with a gauge, so it is possible that is obvously worn, or he has a very good eye, or he actually wants to sell you the whole kit. Besides, the need to change everything together is surely NOT true, since the expected is to change the chain often, so as to preserve the gears, which can last MUCH longer.

I would do the following: buy a new chain and try. If it works, it works. If it skips, I put back the old chain and keep the new one at home. When the time arrives, I change the cassette and then install the new chain.

(Finally, some crafty people are able to remanufacture the shape of the cassette teeth by disassembling the cassette and correcting the teeth shape with a round file, one by one. I did it successfully a lot of times, but it takes some skill and a lot of time and hand labor, sort of a masochistic hobby, but anyway it works and can save you some money or at least the trip to the shop.)