Certainly, they are still valid for postage. You just have to use enough of them to equal the current postage rate for the envelope or package you are mailing.
10 cents.
There was never a 12 cent stamp for regular postage. It went from 10 cents to 13 cents on 12/31/1975. There was however a postage card stamp for 12 cents in 1981.
10 cents
It is 10 cents.
10 cents
Scott number US 1549. You can purchase a copy in new or used condition for 20 cents. It can still be used for 10 cents worth of postage.
You will need to consult the catalogs. Most libraries will have a copy of Scott's Postage Stamp Catalog of the World. First of all, you have to identify the specific stamp or card. That in itself can be a daunting task, but most of the catalogs are well illustrated with lots of information. Once you know what item you have, the catalog will provide a set of prices that can be used to estimate value.
The cost is 98 cents. You could use two 44 cent stamps plus a 10 cent stamp. You can buy a 10 cents or a 98 cent stamp at the post office. If all you have is 44 cent stamps, you would have to use three.
Christmas stamps were 10 cents in both 1974 and 1975. In 1975, the PO wanted to raise rates and hoped to get approval to do so before Xmas and so they left the denominations off the 1975 Xmas issue, but they still sold for 10 cents and can be used for 10 cents as postage today.
14 cents for regular post 10 cents for a postcard
44 cents.
ten cents. They are common.