Avoid using the undenominated 1975 Christmas stamps and A, B, C, etc. That goes even for domestic mail unless you present letters at the counter and have the Postal Laws & Regulations page to prove they are usable.
Any stamp since the Civil War is valid at face value, but why would anyone that is a collector wish to do that! By the way I have a letter with the Cert. Mail .15 stamp on cover and with others sent within the last 20 years!
Also, precancelled or bulk rate stamps can't be used for postage without the correct permit. Special handling or special delivery stamps are no longer valid, as well (they could only be used to pay the fee for those services). But with very few exceptions, anything issued since the 1860s is still good for postage.