In my personal opinion, there is no reason not to clean items which are held purely for their bullion value. Obviously you wouldn't want to go cleaning a coin with numismatic value greater than the metal value, but for bullion I say go for it. even more so with bars and such.
I pick up dirty Maria Theresa Thalers all the time and they each get a clean to make em shiny again as they are only worth the metal value and I like em shiny.
Controversial I know, but I find the cheapest, easiest way to clean tarnish off silver without damaging the surface to any great degree is to use house-hold cleaner called Cillt Bang (called Easy-Off Bam in the USA).
http://en.wikipedia.org/wiki/Cillit_BangJust spray on, wipe with a damp cloth under hot running water, pad dry and done. In all but the most tarnished of coins I find the "cartwheel" effect remains intact post-clean.
Great for copper rounds too.