Check ahead of time!
This has been a few years ago so if it has changed or my memory is fuzzy someone correct me.
I bought my truuck in Tx when I was in Grad. School in Huntsville. I paid state tax at time of purchase, about 5%-6%. I live in Louisiana and registered it here. Our taxes are divided between state and parish (county), which totals 8 1/2%. The state taxes from Tx transfered to the state taxes for La, but not the parish. There was about a 2% ($500) difference that is lost when it crosses the state line. The salesman was telling me about this before I bought my truck. Apparently a couple from louisiana had bought a truck a few years earlier and tried to get their lost money back. He was a lawyer and she worked in the parish court house and they couldn't find where it went either!
Todd