What flavour of BMW is it? They've got quite a few with staggered tire fitment which makes rotating a giant no-no. Even with the same size tires, rotating can be problematic as time in the back can impart a conicity to the tire making it want to turn in towards the center of the car instead of out. This doesn't play well on some models and can result is a sort of twitchy darty feel to the steering. If you know a good BMW guy (I only know one, and he isn't cheap) that can be dealt with but for the price, what are you really saving?
On the MDX, 5k to 8k is probably a good tire rotation interval. I'd go with 5k if you are doing it yourself or get it done at low cost. 8k is more appropriate if you're paying retail for the service or are using tires that you expect to last more than 75k miles. Although some AWD setups are much more tolerant, many AWD cars especially like having even tire wear and the MDXs (the older ones more so than yours) can behave very badly if you don't rotate.
For an extreme example from my personal experiences, but not my car: Cadillac SRX AWD, transfer case grenaded at just onder 30k miles. The warranty claim was denied as the cause of failure was deemed to be improper maintenance. The tires had been rotated once (at a Cadillac dealerhip, but not the selling dealer) at the first oil change and not at all after that.
Another extreme example, not me: A friend working at a Dodge dealership replaced a clutch on an AWD Stealth with a no-go problem. It still wouldn't go, but a 20 minute phone call with a Dodge engineer found the problem. Running too long on the original tires without rotating made them sufficiently uneven as to burn up the viscous coupling. Four tires and a viscous coupling on the owner's dime got it going like new.
