To tumble or not to tumble, that is a question that I consider a lot. Obviously the answer for key-date coins is a big NO but clad and other finds, I tumble all the time. In fact when I have coins that I can't see the date on no matter what I do, I will tumble and sometimes I find out after tumbling that I actually had a key-date, or semi-key-date, that I didn't know I had. But that is ok because it wasn't worth anything when I couldn't tell what coin I had in the beginning. I do everything possible to identify a coin without cleaning it but if after I do I still have no idea, I will tumble for a short time, maybe 1 hour in a few drops of Dawn dish detergent and a dash of baking soda. Just enough tumbling to clean them up for spending.
On a number of occasions I have found V nickels & Indian Head Pennies that I didn't know I had.
I also tumble jewelry and other metallic finds like, silver rings, necklaces, pendants, etc. with fantastic results. Here is a Pandora Bracelet that I found and tumbled with good results.
Be very careful on items that have paint on them because you might tumble it off if you go to long.