There is a risk to not registering, but at this point a seemingly small risk. Not registering could possibly throw up red flags if one is being background-checked for an unrelated reason. The laws mandating the SS were only actively enforced for 1980-1986: http://hasbrouck.org/draft/prosecutions.html
. Add to that, the draft has not been implemented since the Viet Nam war (before the SS existed). The US war machine has been running two simultaneous wars for years now on a "volunteer" army. In my opinion this is so that the ranks can be filled with the poor instead of young adults from the war-declaring classes. Thus Iraq/Afghanistan (with some exceptions) have not been in the public consciousness as much as previous wars. I have heard from several vets about the culture shock they have when they return from deployment and realize no one is paying attention.
One might feel that not registering is worth the risk to prove a point/ as a matter of conscience. In high school and college i had male friends in the anti-war scene (mostly non-@) that refused on principle and others that did it anyway. At this point it doesn't really seem to matter much either way, but who knows what some future hawkish administration might do? For all we know a new war could erupt any day (so many hawks in USA, Israel and Iran want to go at it). Wikipedia describes SS as a "contingency plan". As anarchists we should bear in mind that many government institutions once initiated as a contingency have become part of the regular workings of the state bureaucracy. But, as Taigarun pointed out, even if a draft-based war started you could just run away to Canada, or whatever.
Anyway, meandering sort of answer, but i hope i explained the differing rationales OK. Personally, i don't feel too strongly either way.