I've been assuming that one of the aims of tags (i.e., keywords) is that there would be a search window in the browser so you can search for relevant help patches/tutorial patches/ etc. Or maybe an actual patch, since Hans has said that the keywords should be "parsable in pd." There are already some categories from which simple, standard keywords may be used, but for some terms like "frequency modulation" there's the aforementioned problem of the space between the words. Given that, frequency_modulation is certainly one solution, and users can get used to using underscores when searching, but why not also include "fm" and "modulation" in case the user happens to type that (which would be completely reasonable)