There is a distinct law on this. Though there are different pieces of information you can find about a specific date. In this case, since you're getting the vehicle from BMW, it's best to heed their published information.
In the "Travel Updates for 2011" brochure, that was sent to me for my January 2012 ED, it reads: "Winter Tires - As of the end of 2010, Germany requires that all vehicles must have winter tires or all-season tires (M&S tires) during inclement weather from October 15 - April 15. Please keep this information in mind if you have ordered a vehicle with Performance tires. ....(more)"
I have several German-born, German-residing friends, and they all-to-a-man reminded me of the law prior to my arrival. It seems that the natives (public) is very aware of this as well.
I rented winter tires. From reading postings here, plenty of people do not rent them. It's your choice, of course, as you assume the liability and risks based on your personal situation.
When I arrived at the BMW Welt, I had to sign a document that indicated I understood the law, and that I would assume the liability should I be in a collision and it was found that I did not have winter tires. (I took a quick glance for this document to scan and attach, but those files are off-site right now.)
What I am not clear on, is the
enforceable approach on the law....it seems that if one was stopped for a traffic violation by the Polizei on, say, the 20th of October, without winter tires, that you would not be charged with an offense. But that if you were in a vehicle collision, whether you are at-fault or not at-fault, you would be cited for not having winter tires. Again, there's some murkiness about this, in regards to stories by internet postings, and urban legends.
I think the amount of money I paid for winter tires was a bit high, but it was more like an insurance policy. As it was, it was only the first three days of the eight-day trip that I ran into snow and ice; the remaining five days were all about above-freezing rain. The tires were splendid, and never during all the high-speed (avg 90 mph) rain-driving did I feel the car hydroplane. That money spent on the winter tire rental is pretty much gone and forgotten -- and the car is in perfect condition. Insurance policy....
Have fun! It's going to be a blast.
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http://www.garmisch.army.mil/info/Safety/Winter Tire Safety Information.pdf
http://www.german-way.com/ice-snow-tires.html