The answer, really, is never.
No one wants to see your guns. No one wants to see your tribal tats. Or how well you fill out the jersey. You're not playing in a pickup game. You're not going to impress anyone with your biceps. And if you're not in shape? No one wants to see your arm fat folded over the fabric, or how the jersey accentuates your stomach.
And I'm not just saying this because as a kid in CYO I wore an undershirt to hide my toothpick arms. And I'm not saying this because I look absolutely ridiculous in a jersey these days thanks to my gut. It's just not a good look, especially when you raise your arms in triumph over a LeBron dunk and your neighbor is subjected to whatever romantic smells emanate from your pits.
However, there might be one exception, as I discovered the night the Cavs took on the Suns at the Q. This guy was sitting in the row ahead of me, and he was sporting some serious Cleveland sports ink on his arm: Cavs, Browns, Indians, and even the Buckeyes were all represented.
So, I guess there is one exception to the "No jersey without an undershirt" rule: If you have your team's logo, or something having to do with your team tatted on your arm, then you're allowed to proudly rock it. Just either keep your arms down.