Chuck W, thanks for the input. I usually change out all the fluids in my car once a year, engine oil is an exception, which I do about every 3 to 4 months. The only one I haven't touched is the transmission fluid. Years ago on my previous car (not a BMW, but another European make), before I was mechanically savvy enough to work on my own cars, I had sent that car to an auto shop to do a transmission flush. This ended up costing me a new transmission. The gears kept slipping after that and it never shifted properly again. Apparently the shift solenoids were stuck.
Hence my indecisiveness to touch the fluid on my E90. I'd like to keep the transmission healthy of course, but am wondering if the mileage is too high now that I'd be doing more harm than good by replacing the fluid. Most comments I've seen post-change have been positive ones, but those were also on cars with much lower miles.