I'll probably do it preemptively on my 128i - it spent most of it's life in Austin TX, so high heat and heavy traffic. It's at 49K now, I put on about 5K a year usually so maybe at 100K?
I'm not so worried about it on my '11 328i wagon - always been in Maine, has rarely ever been in traffic. 48K currently, and at <2K/yr the thing will probably last forever.
Interesting comparison of the two cars - despite similar miles, the 128i has had both the oil pan gasket and the valve cover gasket done already, but not the OFHG. No leaks whatsoever on my 328i (bought it new). I just had all the undershields off to fix the lower grill after hitting a bird so I took a hard look at everything underneath. Dry as the Sahara. Also interesting, the wagon gets MUCH better fuel economy than the convertible (both 6spd). Aero? N52 vs. N51 thing? I dunno?
And if anything, I drive the wagon faster - it's quieter.