Honestly I'm not sure if this is common knowledge or no, but here goes.
Let's say an AST want to CO his sexy sexy AoE Balance, I've seen the below:
1) an AST throws up his AoE Balance, and then immediately follow up with CO.
2) CO is on CD, the AST waits until CO is off CD, and then goes to step (1).
I would say the above playstyles is not optimum.
Card buffs are anywhere from 30 seconds to 75 seconds, as long as you use CO anywhere within this time frame, the buffs will still be extended by 10 seconds.
As an AST, you should try your best to fit as many buffs as you can for the CO.
For example, you used your AoE buff cards at 0 seconds, you know you'll want to use CU at 20 seconds, and Synastry on the MT at 25 seconds.
The best course of action is to wait until you have casted your Synastry before using CO, this allows all three of your buffs to enjoy an extra 10 seconds, whereas if you follow the playstyle mentioned above, only your card buffs is extended, not CU and Synastry.
Extra Trivia and Tip:
You're pre-drawing before a pull, you got an arrow, you think to yourself, "whatever man, I've been drawing for so long, this is better than nothing", and you Spread it.
Now you're fishing for Ewer/Spire for the AoE RR, your next draw........Balance, FYL right?
Well... if your team does not mind waiting another 1.30-2 minutes for the AoE Balance Opener, what you can do is as below:
1) Immediately use your Spread-ed Arrow, this will initiate the 30 second CD on Spread.
2) Wait until your Balance Drawn Duration is below 20 seconds, then hit CO, this will bring it back up to 30 seconds (another fun fact, Drawn duration is capped at 30 seconds).
3) With an extra 10 seconds to the drawn duration now, your Spread's 30 second CD will complete before the Balance Drawn effect is expired, allowing you to Spread it.