The WHO has some pretty decent information packages on the global UV index and protection:
http://www.who.int/uv/publications/globalindex/en/index.htmlThe 4% average increase they cite for every 300m means you should not worry about Munich's altitude, other variations (cloud cover, time of day, season, latitude) are a lot more important.
Also note that the WHO teaches orthodox tactics for sun protection. A quick look at some press releases of recent studies (go to
http://search.eurekalert.org/ and type "sunscreen" in the search box), shows that even if you do wear sunscreen it might not protect against melanoma and some sunscreen ingredients actually worsen the effects of the exposure if not constantly reapplied.