For a traditional math grad school applicant (i.e. straight out of college with a strong undergraduate math background), the SOP probably doesn't matter much. All of the professors I have talked to said that pay barely any attention to SOPs at all because they all read alike. I imagine that the SOP is more important for applicants from a non-traditional background.
That being said, I took care to customize my SOP for every graduate program. For example, the Simons Center for Geometry and Physics at Stony Brook is rumored to have fellowships for incoming graduate students, so I was going to apply as a prospective geometer rather than a topologist. For departments with VIGRE grants, I expressed an explicit interest in working with undergrads. And so on and so forth.