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I have had the experience of postal clerks returning to sender packages with the no die cut stamps. I have a large flat that I saved where the clerk marked the cover stating the stamps were phony.
This would be a case where you should file a formal complaint with your local Postmaster and/or his superior (i.e. Regional Manager). The stamps are valid for postage; if a clerk misunderstands that, he needs to be re-trained. The USPS markets these things for collectors and makes lots of money doing it; for those who choose to use some of the stamps for their mail, no problems should be encountered.
Of course, if you have a cover returned with references about invalid postage or phony stamps on it because of the imperfs, it becomes an automatic collectible to the philatelic community!