) Aliasing it to "safe" may not be a good idea since the general internet sense of "sfw" and the local definiton of "safe" do not completely overlap (the latter being stricter). We don't want suggestive stuff automatically taggesd safe, and we also don't want people to develop mistaken ideas of what is "safe" or not.
Not sure if there's a mechanism on the site to null-alias or flat forbid certain tags, but "sfw" would be a good candidate for that methinks.