Depends on how long of a time span. Things change in PC world, and that affects prices a lot when something new comes out, old stuff is discontinued, cheap for a while, then hard to find. Also things happens like the flooding a few years back in Taiwan I think, that cause HD prices to skyrocket since there was any new HD's being made, so current stock was costly. One major memory chip maker has a big issue, prices jump, so what you budget might not buy what you want 3 months down the road.
Generally though most parts are allfairly compatible so if you buy something, odds are it's still good in 2 months.
I prefer buying all at once so I can put it together. A brand new MB/cpu sitting there for 3 months would drive me nuts. lol.