The problem with not purchasing your components all at the same time is that there often is no way to make sure the components you purchase first actually work. There are times when people purchase some components first, wait a month and then purchase the rest of the components; only to find out one of the first components is faulty and you're now beyond the normal return window. Instead of a simple return to the place of purchase, now you have to deal with a manufacturer's warranty process (often taking weeks)
If you have a way to verify the working status of the components you want to get a good deal on now, then go ahead. Otherwise you end up taking the risk of a faulty component and waiting even longer for your build to be...