I don't think you should be to worried. It is definitely NOT comparable with the instability of a poorly fixed untoned, on the contrary (and even these first 19th centuries salt fixed prints have partially survived...).
Originally Posted by MarkL
Most toning manuals that come with the toner don't even mention the need for fixing, I think it's just an extra precaution. The prints should be stable by itself, if properly toned / redeveloped.
My first sepia / thiourea toned prints, when I was still completely ignorant about the processes going on in the chemicals, are still 100% fine even without fixing more than two years on. OK, that's not so long, but far longer than any time it would need to show a "fixer" type problem.
Most bleach baths contain a mixture of something like potassiumbromide and ferrocyanide. The bleached image actually probably consists of a mixture of silverbromide, and a lot of silverferrocyanide, which is colourless / yellowish a well. The remaining silverbromide after toning in thiourea, is probably not much.