Underpaid or overpaid is not the norm under PAYE provided the employer has followed correct tax code issued by HMRC.
Yes, I know it can happen under certain circumstances, but mostly when it does
it gets corrected.
There is nothing wrong in asking HMRC
My advice regarding tax (and NI) is keep on top of it at least annually if needed.
Incorrect application of HMRC issued tax codes are the responsibility of the employer not the employee
