I treat most promotions as important aspects of the PC's development, including demotions.
Being promoted to Lt.Jr. grade is no big deal. Everyone should be able to make it there (well okay, Ensign Kim is an exception, but all the odds where against him ). Same for crewmen, their promotions wouldn't be something most people outside their immediate entourage would notice. Like getting promoted from Crewmen 2nd class to 1st class.
I usually have it included at the beginning of the next episode in a formal ceremony. Although, I've only promoted two or three PC's to Cmdr. and all of them were done between seasons so I could bring them unto new ships.
Commendation should work the same way.
"The misery of being exploited by capitalists is nothing compared to the misery of not being exploited at all."
-Joan Robinson, economist