I'm pretty new to valuble coins and I've heard a lot about cleaning coins and that I shouldn't but what are the ups and downs of cleaning circulated silver coins and pennies? Thanks
There are no upsides. The downsides are that you will seriously ruin the coin's appearance and value. A bright, "shiny", circulated coin looks unnatural and is not worth nearly as much as a coin with its original surfaces intact. By all means, experiment with some pocket change, though.
Paul hit the nail on the head. There have been several recent threads with excellent advice from the site's veterans. Do a search and you will find them.
I also agree with what Paul said. Additionally, if a coin has PVC residue (or glue on it, etc.), 100% pure acetone (NOT finger nail polish remover) can be used to dissolve the organic compound without affecting the coin. Be careful with copper coins though, because acetone can allegedly do some weird stuff with copper. Acetone is more accurately called conservation instead of cleaning because it's helping the coin. There's probably a lot more details on this in the threads Santinidollar mentioned.
The upside? Your coins will be prettier. The downside? You will destroy the numismatic value of the coin and your reputation as a numismatist. Of course, there are exceptions to the "never clean" rule. Some things (like PVC plasticizer corrosion and verdigris) will damage the coin if left untouched and must be treated. But if it's voluntary, you probably shouldn't.
