If the manufacturer is smart they will put "less than 16 inches", but I have seen some that said things like "10-14" inches. In that case, you can't go less than 10 inches even after the tax stamp is received.
This is not true. BATF likes to know what lengths are used, but you are not prohibited from using a shorter barrel than is disclosed on the forms. If you make a "permanent" change in the barrel length, BATF advises you to amend your F1/F4 and inform them of the change. For weapons like an AR15 where barrel/upper changes can be done in a matter of seconds, almost no change can be considered "permanent". On fixed barrel weapons such as a Ruger Mini-14/ACC556, HK types, or AK47s, you're not going to be able to easily change the barrel length, so the paperwork should reflect the actual length, but if you do change it, you can simply write to BATF and inform them of the change.