If by buy parts over time you mean buy the core part of the build (RAM, motherboard, CPU, GPU, case, at least 1 hard drive, and PSU) and then add in additional drives, peripherals, monitors, crossfire/SLI GPUs, whatever, then sure. But if you buy a motherboard this month, and then a GPU the next or something to that extent, then:
1) You've just wasted money because the parts will surely be on sale or permanently drop in price within that time frame
2) You're outside a 30-day return window for RMAs from the store
3) You'll have a pile of parts that you can't do anything with because you have two whole components
I understand the impatience, but I personally really regret building my rig when I did and with the knowledge I have now. I essentially wasted $50 on a GPU/wasted a potential performance increase, I bought a crap PSU, I should have bought an 83x0... Give it some time and save up, and while you're doing so, read and ask questions. It'll just be that much more satisfying when you build it.
I'll take what you said into consideration, also, I am only 14 and do NOT have a job I make $40 every week, which is why I am asking this question in the first place, if I had a job I would buy them all at once, but unfortunately I don't and I am impatient and cannot wait 6 Monthes to buy it all at once, where as if I buy it overtime, it will be faster and I can start building my PC quicker then if I were to buy it all once in July.
as for your points:
1.) Not really concerned, in the long run I'll make back that $20-$50.
2.) I take good care of things.
3.) At least I can bask in its glory lol