Someone just pointed me to this.
I assume this should be in the same category as other such symbols that glorify violence in the service of bigotry or genocide, such as the Confederate Battle Flag, and swastikas. Specifically,for for the purposes of discussing relevant history, non-gratuitous use of such symbology in a post on History.SE would be OK (eg: a picture of a destroyed Russian tank will quite likely show a painted-on Z), but use in a profile picture or user name would be right out.