For most cultures, I utilize my cultural revenue exclusively. Unless my retinue becomes very, very, large in which case I add in a bit of defense retinues {unless my cultural unit is defensive} and some archers {again, unless my cultural unit was already archer based}.
Cultural units will almost always be the best bet. The question is whether you're able to field armies of retinues, or whether you're small enough that a retinue is an elite aspect of your army. So, in short, if one plays Ireland, they may want to avoid all non-Gallowglass units, but if one unites the Empire of Britannia, then they may want more diverse retinue stacks.