I've wondered about this too, and did several searches when I was starting to do testing, and came up with very little about the effect of glycerine-based substances leaching from soft glass.
So I just kept the silver/metal-laden colours away from the ends, and used bases of black and clear mostly, for the cores. Not sure, though about other base colours like Ivory or red, blue etc, which I've also used for fitment testing.
Having said all that, I have also make my concerns very clear to my beta testers, and encouraged them to do their own research as well. And funnily enough, there seems to be a few chemists who are members of the Aussie forum.
And, surprise, surprise, another thing against the idea of coldworking
(grinding) ends, since I'm not sure whether possible leaching would be exacerbated with the surface compromised. I'm also not sure whether the rough inside hole would be considered the same as a ground surface, given that glass laid on a mandrel merely sets on the clay, whereas grinding actually opens a surface.
I'd be curious to hear more about this, too.
And, for the record, to date, I have not sold any tips, or have put any up for sale. Still working on design etc.