This is probably the website that you were referring to: http://www.thegunzone.com/rkba/rtc-usps.html
If this lawyer is correct in his interpretation of the federal laws, you may carry in post offices unless otherwise prohibited by a state or local law. There is no federal law prohibiting carry in retail banks, so it is up to individual states or the banks themselves to prohibit it.
I took a look at the firearms laws on michigan.gov, and I found: 750.234d, Sec. 234d. (1) Except as provided in subsection (2), a person shall not possess a firearm on the premises of any of the following: (a) A depository financial institution or a subsidiary or affiliate of a depository financial institution... Subsection (2) (c) exempts CCW permit holders from this regulation. I can't find anything prohibiting carry in a post office. I am not a lawyer, but it would seem to me that if you have a CCW license, you would be at the mercy of your bank's policies, and the official interpretation of the CFR pertaining to post office carry.
VA does not have a law against either, so I do carry when I go to my post office. I CC, and pay particular attention to avoid printing, simply because I'd rather not be the test case. I have OC'ed at a couple different banks with no problems, but many of the larger banks in my area have posted policies against it.