The reason a per diem is normally TAX FREE (borquette's emphasis) is because it is used to cover expenses not normally incurred for providing services. It requires receipts, record keeping, and justifications if you want it to remain TAX FREE (borquette's emphasis) and is subject to limits. In other words, it is not TAX FREE (borquette's emphasis) cash into your pocket for you to keep, therefore it should not be considered part of your take-home pay.
Either this borquette is stupid or she thinks you are.
http://en.wikipedia.org/wiki/Per_diem
The factor that makes tax free per diem available as an option in the first place is that you work at a business location that is outside the commuting range of your domicile. That always means that you have hotel bills, travel and eating out expenses.
If you
don't have these expenses, then that means that either you are living in a homeless shelter while collecting per diem (unlikely), or, more likely, you are collecting per diem after you have moved yourself more or less permanently to the vicinity of the work location.
That latter condition makes you ineligible for per diem because you are now domiciled near work.
So, if you can legally collect per diem, it offsets actual (significant) expenses of maintaining extra living quarters. Either an extended stay inn, or a hotel, or a short term apartment.
If you don't have these extra expenses, because (for instance) you moved your family to the new job, then you can't collect the per diem legally (tax free.) If the IRS probes and finds that you had no presence as a household in your old city while you claimed tax free per diem, they will want to count it as wage income from the time that you moved your family to be closer to your job.
Unix, please be clear on this issue. Most recruiting agencies that attempt to explain per diem get it wrong and misrepresent it, because it is no skin off their asses. Per diem is only tax free when you have the financial hardship of split residency.