Did the 96+ OBDII Bonnevilles have two oxygen sensors like most other OBDII cars or just one?

2

One in the rear manofold and one after the cat

I hear some people change both, others just change the one before the cat. Which is the proper way and does it really matter in the end?

The one in the front is the buisness end, the one in the rear from my understanding is just pretty much a check for the front o2 and the cat convertor to make sure they're working properly, or something like that.

Correct. Rear O2 generally doesn't need replacement, though I would replace if:

1) New cat is installed.

2) Catastrophic UIM/LIM gasket failure (i.e. major coolant buring), in which case spark plugs will need to be replaced as well.

3) Computer throws the code for it.

Front O2 will fail way before the rear one does on a well maintained engine. And for the love of Mike, don't get Bosch.