Yes, you can purchase it any time during the initial 1 year warranty period, so you could wait if you want. Of course, you would lose the 9 months of technical support you would have had during that period if you had bought it at the start.
I'm not usually big on extended warranty plans, as they are, for most companies, a way to increase profit. As a result, on average, consumers will lose money on them. On the other hand, portables are much more prone to failure than desktops, and a number of MacBooks have had issues such as faulty HDDs.