How important is accounting for S&T

Completely depends on what asset class you trade.

For FICC, little to none: when I worked in FX, there was no accounting at all. Same goes for government bonds. Virtually the same for securitised products (although this was only a brief stint for me) - was less so about the credit worthiness of the issuer (where I'd imagine there'd be a greater emphasis on accounting) than how the mortgages themselves were actually rated by the regulator.

Have never worked in credit, but imagine there is some simply as a result of the asset class itself literally being corporate debt. The most emphasis I've seen on accounting was when I interned at a quantamental equity fund. Even then, however, the knowledge requirement wasn't huge and I found I was able to get up to speed relatively quickly. I wasn't asked about any accounting principles in the interview.

In terms of accounting knowledge for S&T interviews in general (assuming you're mainly interviewing with BBs), I can never (to the best of my knowledge) recall ever being asked anything to do with accounting. Was all brain teasers, product knowledge, general market opinion stuff.

If you're asking this because you study accounting and think it will put you at an advantage from a technical perspective, sorry to disappoint. If you're not and were simply worried because your accounting knowledge is light, you're in luck.