well since Taz and Hskr seem to know it all, I'll let them deal with it. I only have the FM mod. setup in a vehicle which travels 110,000 miles per year over 4 to 6 states through several dozen cities where there are dozens of upon dozens of stations broadcasting, including several high power military stations, and only have to change the channel in two cities and even then it depends on the hour of the day. Of course this is only in a vehicle inwhich the main antenna is always concealed behind a fiberglass shield so it has to work even harder to receive a signal.
But then again, in 6 yrs of using XM, 5 different XM units in that time I've never had a problem and have only installed without a hitch 10 different XM units in 10 different vehicles.
The typical XM unit with an FM mod. built-in has about a 3 dozen differnt channel choices. As I have found, since most every AM/FM radio on the market has 10 to 18 FM presets, I just pick 2 or 3 different presets on my radio and lock them in, then if I need to adjust, I can check those channels.
Oh and one final thing, even doing the hard wire (think XM Commander) is no guarentee that some FM station won't blast over the signal. That is how our setup in the Audi, and it was installed by the Audi dealer and checked by an independent shop after a problem, and there is still one radio station in Wyoming that broadcasts on the same frequency and will override the XM signal even with it direct wired to the radio. The only way around it 100% is to buy a radio with an XM or Sirius tuner built in.