Rant about omitting o.d.d.s: if you're building cheap pc where very dollar counts i.e. saving $18-22 gets you a better case or gfx card, and you already have a sata o.d.d. in your possession, only then you should skip the o.d.d. otherwise no.
for u.s.b. booting, the first thing you need is the iso of the o.s. to get an i.s.o. you need an internet connection, to use that internet connection you need another p.c. (likely well protected with antimalware software etc), a usb flash drive.. so you see, the iso is nowhere near the first thing you need, the first thing you need is another pc, then a software that turns the o.s. iso disk ready for installing from a usb flash drive. if you have the o.s. on an optical media.....
so what if you have only one pc or no working pc during assembling and installing the o.s.? things get worse when you're using usb-installing from the first time. linux distros play nice with usb but windows doesn't (<- this sentence doesn't begin to cover how much p.i.t.a. it is to prepare windows 7 install from u.s.b. that's another rant. differant, geddit?).
let's assume that you finally installed the o.s. what about drivers? motherboard and gfx card drivers come on optical disks, how are they gonna be installed without and o.d.d.? let's assume you have a way to install drivers directly from the internet. that means connecting your freshly built, unprotected pc running only the o.s. (may be with windows firewall (LOL) and mse(LAWL) enabled) to internet, browse with built-in internet explorer browser, and hope that no one notices you. although, you wouldn't even know if something got into your pc.
i don't know about others, but i'll buy a sata dvd writer.
this was about self-buillt pcs. devices like netbooks and ultrabooks often necessitate re/installing from usb.