Should I cash or deposit my 401 K checked I received in the mail?

[VolvoGirl]

Either way.  What do you want to do with it?  Buy something?  Do you want to roll it over to a IRA or another 401K account?  I would first deposit it.  Then you might not spend it quick and can save it.

[Opus 17]

Why did you get a check?

If you are doing a rollover to another account, is the check even made out to you?  (It might be made out to something like "Vanguard in trust for John Smith".  You can't cash that, you just sign it and forward it to the broker where you are doing the rollover.)

[dmertz]

If the check is made out to a financial institution for your benefit, it's not payable to you personally so there should be no need for you to endorse the check.  As Opus 17 said, such a check can only be forwarded to the financial institution for deposit into the receiving retirement account.

If the check is made out to you and you do not roll the funds over to another retirement account, you will be taxed on the taxable amount shown in box 2a and, if you are under age  59½, you'll also have a 10% penalty.