Prices tend to fall over time. So assuming the parts will be available in the future, better wait. Prices go down. Moore's law for one, but also because new stuff is being brought out.
Then there are always deals to be had. I got a "25% off" deal on a case, a PSU, a Quadro graphics. That one looked amazing at the time (a year ago). The case price is slightly lower today than when I got it, the PSU can be had for about 40 bucks less now, the Quadro is still, surprisingly, about 50 bucks more than I paid. I got 50% off on three GTX580 3GBs. I really wanted the 580s rather than the 6xx generation. Of course the 780s which were coming are better, and prompted the dealer's sale, but price for performance wise I still got a good deal. In fact the deal was a little too good, since they could only muster two cards. They took forever to deliver, though. Three months. I got a refund for the third, but would have preferred to have gotten all three. I wanted a particular motherboard which I thought might not be available in the future. It can be found, but for more than I paid, and there aren't many left. Waiting is worthwhile assuming all parts will be available, or if you are flexible. The water block for the motherboard was hard to find, too. I got lucky. However, my build isn't finished yet because I haven't been able to get hold of all the parts, and some deliveries have taken forever.