Like anything, it depends on the specific circumstances and there is no one right answer.

If it's a company/organization that you've done business with, and they got your phone number through legitimate means, replying STOP will unsubscribe you, just like clicking unsubscribe in the email will simply unsubscribe you.

Folks saying that the amount of political spam they got after replying STOP to a political message went up, probably submitted their phone number to more organizations than they realized.

If it's actual spam (Which in my experience, is more often email-to-sms from Gmail, not actual texting spam) then of course that doesn't work.

Unfortunately this is a situation where you just have to use your brain. Oh, and I'm not convinced "report spam" does anything.