Hi Brian,
If it makes you feel better, then clean it, but it's not really necessary. The tarnish is only a film of oxide that the flux will remove.
When doing a brazing demo, I might have cleaned the rod purely for a esthetic reasons.
I would use a piece of newspaper. If the silver solder contained more than than 40% silver I would use a broadsheet, if it was less, I found a tabloid satisfactory!![smiley smiley](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
Regards
Keith