Apart from buying a new set of 4 with a set of aftermarket wheels on my Golf GTI, I've always bought 2 tyres at a time and put these on the rear whilst swapping the part worn tyres to the front (irrespective of driveline). Mainly for the following reasons...
1. It's what's recommended by most tyre manufacturers (to alleviate the problem of the most amount of grip swapping ends)
2. You end up buying 2 tyres at a time rather than 4 - whilst I could afford to do so, I'd rather not have a £600 bill in one go on top of normal servicing etc.
That said, I've never had a TPMS system that uses sensors. All the VAG one's I've had use the ABS sensors to measure wheel rotation speed so it's as simple as resetting the system.