Well, its not completely necessary to change both if only one is flawed.
But you really do want both your left and right sides operating at the same specs/rating. You don't want one side stopping harder than the other. It really is safest to change both at once (when it comes to sides).
I think its ok if you do just front or backs...
exterior:Tenzo DC5, blacked emblems, mugen grill, 25% tint, pwp hood trim, blacked calipers, front/rear A-Spec lip, 06 sides, spoon replica spoiler
performance & suspension: Apexi WS2 Catback, magnaflow cat, Ralco RZ pulleys, TS header (wrapped/sprayed) & vibrant flex pipe, cylinder resonator removal, mobile 1 synth, K&N drop-in, wicked tuning camber, Tanabe DF210 springs, koni yellow struts, comptech RSB, r1concept e-line rotors, akebono proact, DRH tb spacer
lighting & audio: Audi & blacked (housing) headlight mod, 5K low beams/HID fogs, smoked tails (spray), laminex, LED cabin, piaa reverse&license, aux link, 10" infinity sub, 1000w MTX amp, Polk DB651 (all doors), LED cupholder