Do it because it makes sense.
I changed mine at 12k (1 year old). It was nice and pink - just like I wanted it. I didn't flush, but I got about 9. 5 quarts out by dropping the pan and changing the filter. I plan to do it yearly - whether it needs it or not. It just makes sense to me to keep it clean. By doing it yearly, I figure a flush won't really be necessary since I'm cycling new fluid through it relatively frequently. As I see it, at about $45. per change, it's cheap insurance. I also installed a temp. gauge with the sender in the cooler line.