there are actually only few regions that profit from a lvl 5 regional barrack. as mentioned above every faction has its own selection of regionals. slingers generally don't need high level MICs you're on the safe side with a lvl 3 gov but most times a lvl 2 would be ok aswell as most regionals only need a lvl 3 MIC.
check the Recruitment viewer to see what your faction can get on level 5. most notable examples are the aforementioned balkan regions and India which is home to indian elephantes
just checked and many factions only have the tracians the scordici and an other unit on the lvl5 regional barrack