I usually don't give any promos to fresh units, unless that unit will be used immediately in some capacity for defense likely.
Promos equal health and are best given when you know the intended purpose, whether offense or defense, so saving them can be a good move.
City Raider (CR) on Melee/Siege for attacking cities.
Support promos for some units on defense or attacking in the field.
Generally combat on Mounted and maybe a couple of Shock promos for Melee/Counters.
I do really like the Accuracy promo on some cats early on, if I have the luxury/success to do that. I tend to keep those alive if I go the Treb route later so Trebs can just get CR promos. I'm more of a Mounted warfare guy though, so siege is not often what I use.
There are certain exceptions for certain leaders or uniques. I like the Drill line on Chokes, Oromos, and SB LBs if I have the luxury to play around with that...and even Samurais.