Well, I like to balance it out a bit, as I go, but I seem to really like the Pikes/archers, as they are usually underrepresented in your levies, they are only a little behind your heavy infantry on the shock phase attack, but in defence they excel, so very useful in the baiting of enemy AI and then you flank them with your Heavy cavalry retinue in a neighboring province. I primarily use a bit of heavy infantry to flesh it out still though.My ratios tend to be around:
Defence(Pike/archer):2.5
Cav:1
Shock:0.5
Of course substitute your cultural retinues(italians and scots have their own pike retinue, or use Hussar, Cataphract, Horse Archer etc. based on personal preference in the cav slot).
I really only use the Skirmish retinue when I can't afford something better and need it fast, and people that want archers already get the longbow retinue anyways.
EDIT: I'm not sure if you get the bonus from cultural buildings but the cultural retinues do get unique bonuses:
RETTYPE_CUL_BYZ =
{
first_type = 4
first_amount = 300
second_type = 6
second_amount = 200
culture_group = byzantine
modifier = {
knights_offensive = 0.3
horse_archers_offensive = 0.3
}