Yes, AR holds the assignment of the tax groups to the customers. The actual purpose of the tax groups in AR is to match with OE.

If QIP is not 'reading' the new tax group (created new in QIP to match the new one in AR) when entering a new invoice, then you could check the following to ensure there is not something else causing this to occur:

1. Try exiting out of QIP and AR and going back in. Perhaps the files involved were not updated correctly to reflect the changes.
2. Ensure that the actual QIP tax "type" categories have not changed vs. the AR Tax Code information. For example, if AR is setup as:
0. Tax Exempt
1. GST & PST Taxable
2. GST Taxable only
3. PST Taxable only
Then this must match QIP to be effective.

Also, they will not be able to 'save' the changed tax information in QIP if:
1. There are open invoice batches
2. The Tax Summary Report has not been printed and cleared.

Hope this gives some more direction...

Good luck!