I use true headings, not magnetic, and on VHF-UHF it can make quite a difference. I doubt anybody has a 40 meter beam sharp enough where it matters...
Where I am in L.A., we have a magnetic declination of more than 13 degees (east) at this time. On 70cm where a long boom Yagi can have a 9 degree beamwidth, that's quite a lot of error. On 20m where I have about a 45 degree beamwidth, it's just about nothing.
But "magnetic north" is constantly changing, so setting up by that today is quite different from setting up for it a year from now. True north stays in the same place.
If you don't know your specific declination, you can find it on line, continuously updated, here:http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp