Are you certain of that?
In most states, drinking alcohol and tobacco are taxed and must have stamps affixed to the bottle or cigarette package (at least from memory, I haven't seen either in 10 years.) Does Officer Friendly have the authority to see if the package of smokes in your shirt pocket is wearing a properly affixed tax stamp?
http://www.ar15.com/forums/t_6_17/367276_.html
It is not necessary to carry the actual tax stamp along with the NFA item but it is necessary to be able to conclusively demonstrate that the tax was paid and the stamp issued. The only folks who need to see the actual stamp are BAFTEIEIO agents and they cannot require you to produce it on the spot. Some states have laws that prohibit possession of NFA firearms "unless possessed in accordance with federal law." Non-ATFEIEIO cops in those states can demand to see proof that your silencer is possessed in accordance with federal law in order to determine that a state law violation has not occurred.
As to your specific question - while Ossifer Friendly usually is not be empowered to investigate whether or not the federal tax stamp for alchohol or tobacco is affixed, he can investigate whether or not the state tax has been paid. The only way he can determine if the state tax has been paid is by seeing the federal tax stamp which is affixed only after the state tax is paid. Sort of round-about, but as long as he stays within his authorization to enforce state law and does not venture into trying to enforce a federal law he is not authorized to enforce, he still gets to look at the stamp.
Easier to understand than a used car warranty, isn't it?
stay safe.