Hello there, Great question. First off when you are advancing merch for the tour you can ask what is the sale tax in that city and who retain the tax (venue or band) that way you can make the adjustments on your sales sheet. In most cases it will be the band submitting the tax. Which means you retain the cash and deposit it with the merch sales in the bands account. When you send your show sheets each night you should have recorded the amount of tax and if the venue or band kept it. Some venues keep the tax and submit in your behalf. After that you have done your job its up to accounting (if the band has one he or she will submit the taxes) Your job is to make sure its registered in your paper work.
Good luck keep on doing the right thing.