If you have an important document relating to a dispute with your employer- for example, a termination letter or a pay stub showing underpaid wages- please save that document. (You can read more here about keeping good documentation).
Just as important, please do not write on the document, or otherwise alter it.
Keep in mind that important employment documents may later be used in legal proceedings. For example, if you want to use your termination letter as an exhibit at an unemployment hearing, you don’t want to show the judge a letter that has your added, handwritten notes across it, saying things “This is a f#$% LIE!!!!” (I only exaggerrate slightly- I’ve had clients who marked up documents with notes reflecting their frustrations in similar terms).
Bottom line: hold on to important documents, and don’t mess with them.
WI Employee Rights Lawyer: Severance, Wages, Job Termination, Discrimination 
Is a termination letter required when one is fired? Or at least a reason for termination?
No termination letter or reason for termination are required, unless your employer has a policy or contract that requires these things.
One exception of note: for those employees lucky enough to be working in Montana, in that State an employer must have “cause” for termination. So, under that State’s law, an employer must have a legitimate reason to terminate someone’s employment.
But for those of us in the other 49 States, our employers can terminate our employment “at will.” That is, we can be terminated for just about any reason (even for unfair reasons), or for no reason at all.