The 220-age formula is an average ( and even at that is suspect) and can be way off for any particular individual. For all practical purposes its useless. For some interesting history on the genesis of the formula and it's limitations see this (you may have to register with the NYT to read, but it's free and worth it).

If you're in good health, I don't think there is much risk in doing a self test. YMMV. Obviously, if you develop chest pains stop and seek medical attention. Be forewarned tho, a test of MHR is going to be uncomfortable. However to effectively use a hr monitor, you really don't need to know your MHR. Your hr at lactate threshold is more important and useful. The Friel and Carmichael books each have ways of testing for this and give you workouts based on that number. This is a sub-maximal test, so doesn't carry the same risk as a test of MHR. There is also a sticky in the Training & Nutrition forum that gives a threshold test. There have been numerous threads there about HR training, so if you do a search you'll find a lot of info.