The way RAM prices are right now, it's probably cheaper to just have Apple install it - it's going to cost on average the same anyway. The advantage of having Apple install it is that you won't have extra SODIMMS lying around, the RAM will be covered by Applecare, and should you ever have to send the computer in for service, you won't have to swap the Apple RAM back in.
If you search the forums you will find instances where people have sent their upgraded machines in for service, only to have them returned in the stock configuration - allegedly this is what Apple does when they restore a machine back to stock condition, but I can't confirm that.. I don't know if this is how it's done in general, or if the people in question were just unlucky, but it's something to consider. I was planning on selling my stock 2GB of RAM, but after reading these anecdotes, I'm going to hang onto them for a while at least.