I've always used realistic army compositions in EB. I try to use them in all other strategy games as well. When playing as the Romans in EB I'm fanatical about realistic army composition because we have very good sources telling us exactly what their armies looked like. When playing as Seleukia I use a core of good quality phalanx troops and a small number of quality heavy cavalry recruited from Antioch, Seleukia and any other core Seleukid province, then lots of light troops, light cavalry and slingers and archers from wherever the army passes. If I'm fighting against the Parthians I'll draw lots of slingers and archers from those eastern provinces. Stuff like that.

Since I don't know how to compose a realistic army of "barbarians" when I play as someone like Averni, I tend to use a core of decent swordsmen supported by tons of light spear armed infantry and mostly light cavalry with one unit (usually the general) of heavy cavalry.

So yeah, in general I always use realistic armies if I can.