My personal preference is to use it at the peak of my fury bar for max damage before starting my attack chain.
It's the exact same numerical increase in damage no matter when you use it. Say you have an attack that does base 100 damage, enhanced to +95% damage.
195 damage w/o BU
275 damage w/ BU - increase of 80 damage points
395 damage w/ max Fury w/o BU
475 damage w/ max Fury & BU - increase of 80 damage points
So it doesn't matter when you use Build Up, since you're getting the same net gain of damage output. All that matters numerically is what attacks you manage to use during the BU period.
Issue 16 made me feel like this.
Warning: This poster likes to play Devil's Advocate.
I prefer option 3: At the start or the end of the fight, whichever point you're hitting that thing that needs to die NOW. Particularly if it has high defense or to-hit debuffs.
Having Vengeance and Fallout slotted for recharge means never having to say you're sorry.
I pop BU whenever I have lots of enemies around so I get the most out of AoE attacks. Or when I need to finish something fast. Or if it's recharged and I don't mind waiting 25 seconds to get it up again.
That makes sense. Enhanced for recharge, I have it up often enough to use it in both situations when I remember. We were just curious when the extra burst of damage is more needed.
Thanks
I often used to use bu and FE on my em/fa to smooth out the lack of fury at the beginning of a mission (popping both is like having 80% fury) cause once fury was up I could crush everything. That was of course back when the toon was fun to play and wasn't deleted heh.
The other good use for buildup is when you want something to die 
Just a strange question someone looking over my shoulder while playing asked the other day:
suppose you don't slot build-up, and it has it's default recharge time. This means that it will be available only once in a fight. Should a brute make better use of it...
1) At the beginning of the fight to make up for lack of fury?
2) Mid fight once fury is built up, to do more damage on top of the high fury damage?