So you would not recommend replacing O2 sensors without codes, even at high mileage?

My new-to-me 169k mile X5 with lots of repairs and not so much maintenance shows no record of O2 or spark plug changes. I'm doing the plugs (just like I'm on my way to doing all the fluids), but I usually only replace O2 sensors when there's a code. No running problems except that gas mileage on our first tank was only about 12 mpg (all city, 2001 3.0i). If I knew for sure that the O2 sensors were original I'd replace them. But there's a good chance they were replaced already and I just don't have the records. Also, the car has blown extremely clean smog check results for all of the 3 tests that I have records of.