I did some POS work for a company years back and tried to integrate 'taxes'. I was replacing an earlier system, and I used modern tax SaaS (avalara, IIRC). The client was telling me all my taxes were wrong, but... digging in... Avalara was correct, and the client had been calculating and collecting taxes wrong for... about 15 years. I had no way to try to figure out how they were doing it 'wrong' to match how they'd been doing it. This had to do with 'service/labor' income vs 'product' vs 'chemicals' they used. Different counties in the same state had different rates (which changed over different months/years historically). Tribal lands had different rules still.
The biggest lesson I took from that was... if you do something (tax calculations), do it consistently and uniformly. Should they have been audited, it may have looked a bit better that they were at least consistent in their tax issues, instead of looking like they were explicitly trying to somehow intentionally scam customers or the govt.