So been riding for years a 7800 crank/bb combo on my bike and always thought it was nice how snug the left crank arm was to the BB. Building another bike with a 7800 crank/bb as well, but noticed how there was a very noticable gap between the left crank arm and BB. Hmm, mis-milling? Well took the cranks off and tried both left arms on both bikes. The one fit snug and the other had the 'gap' on both bike's BBs. Examined the arms. A-ha. The one with the gap has a gasket type o-ring of sorts at the end of it thus limiting it travel as the BB has a slight taper to it. The other is missing the o'ring thing and gets nice and close. My question to the audience is: Do I need this o-ring? Thanks.

No. It's a spacer to be used where needed.

But worry less about shell clearance than Q. Q is the distance between the centerline of the bike (say the center of the seat tube or down tube) and some spot on the pedal (say the outer flat of the crank arm). It should be the same on both sides. Since the drive side has to have a place for the rings, if the arms have the same flare then the root of non-drive side arm will have to be spaced away from the shell.

But there's really no way to tell if the arms have the same flare, so you should measure Q on each side and use the spacer accordingly.

Thank you.

Be sure to read my edit. You might have read only the first line which I posted too quickly.

Ok. Out of curiousity, is this more important for the rider or bike?

Totally for the rider. Riding with a badly mis-matched Q would be like standing with most of your weight on one foot all day.

What is the measurement procedure? What range if not perfect is acceptable/not acceptable? Thanks again.

Measurement procedure is implicit in my post above: some tube centerline to some point on the pedal or pedal-end of the arm. Same points both sides. You're not interested in the absolute values, but in the difference. There's no firm rule; some legs are more sensitive than others. And you only have two options here so just choose the one with the smaller difference.

Got it. Thanks.