I only bill for provider, but specifically for add on codes, there was a new guideline starting 2020 that -50 should not be for add on codes. Add on codes should be -RT and -LT.
That being said, as someone who bills a bilateral add on code several times per week, I don't think any of the carriers updated their software for this change. From what I have seen, some carriers always wanted -RT -LT whether add on code or not. If the carrier previously paid as -50 and did not require -RT -LT, they still don't require it for add on codes.
I consider it payor specific, regardless of the CPT guideline for -50 on add on codes.
But the "correct way" for add on codes is only using -RT -LT. For non add on codes, it is carrier discretion. I personally bill -50 unless I know in advance carrier will only recognize -RT -LT or if claim denied with -50.
Hope that helps.