I guess the cloth is my best option unless someone knows a secret
formula for a dip that doesn't make the tinsel re-tarnish too quickly.
My worry is that if I clean the tinsel and then tie up some Salmon
flies with it. The tinsel will re-tarnish and detract for the look of
the finished fly within a week or so.

I appreciate that tinsel will tarnish on flies anyway but this is
usually quite a long term process of years rather than days or weeks.

Well, you can coat the tinsel with nail polish just before applying it
to the fly and let it dry thoroughly prior to wrapping. That's what
I've done for some shadow box Salmon flies that I've tied. I've also
used a VERY THIN coat of softex, which remains flexible after it dries,
but I'm a bit concerned that it will yellow over time.

Larry