Not really, but in buying dual channel memory kits is the way to go -- because you need to buy memory in pairs, thus there are these kits with two sticks of memory. Supposedly matched.
Buying the kit means you'll be getting memory that'll run at the speed marked on the package.
Buying two individual sticks means the memory will most likely have to run at the slowest DIMMs speed.
Not a big deal on the Mac, but on a PC where you can tweak the settings you might forget about the slower DIMM and end up with an unstable DIMM -- corrupted memory/HD, unstable machine, etc.