I changed my Bridgestones at the same time I change any set of tires on any of my my cars. Never let them go down to 1.6 and just change them when the wet performance starts to suffer... co-incidentally though I think that it was around 4mm on my original Bridgestones. For me it depends on time of year. Summer I will run them a little longer as it will be mostly dry and good temperatures and lack of standing water in general... winter, if I get a sense of skittishness at all in the rain I will just change them.
Might be different this time as I am trying to time it to change 4 tyres at the same time in order to get 235 width tyres on to go with the wider wheels. I prefer sidewall to the stretched tyre look but my tyres had a lot of tread when I bought the wider BBS and the 225s were the width most sellers packaged them with. Will see if I can rotate them for even wear so I don't have to get rid of rears with loads of tread.
On my 3 series I used to change the rears at about 6mm. There was a roundabout on the way to work that with the camber and rain caught a lot of people out even at low speeds. I knew to take it carefully so when the traction control light came on there was when I changed my tyres.
