Does the tarnishing reduce the value of my coins / bars and should I polish the silver?

No. As for scratches the intrinsic value of bullion is not affected by tarnishing, however if it is important to you, there are ways to greatly slow down the tarnishing process and remove tarnishing. However, from a bullion perspective you should not aggressively nor repeatedly polish coins as the process can remove the outer layer of the coin/bar which causes a small loss of silver. This will make the coin/bar lighter and therefore reduce the value of the coin.