It's no big deal unless you do it with the intent to defraud someone when you spend it,
or shave it to keep the 'extra' for its bullion value then spend it for full value...
Or making a coin look mis-struck and more valuable.
Or change a dollar bill's value such as putting corners of a $5. on a one dollar bill.
"Intent to defraud" is the operative part of the law.
When the money is in your possession you can do what ya want to it, just don't try to spend it after you deface it.