Hi Japester,
yes, cavalry receive attack bonus when in flanks, and is the stronger unit for field battle.
but cavalry is strong in the center so long as it doesn't face a wall of pikemen
for example, in v6 version of the game the % of cavalry in the flanks will depends on your army composition, but also the Battle Formation.
the default setting will have 20% of your melee troops in flanks, while Strong Flanks formation will have 40% of said melee troops in flanks
but in v4 and older version, 20% is the optimal size to maximize your cavalry
that said, it doesn't necessarily equate with stronger army ... for that you need to take your opponent's army mix into consideration (and the terrain) : plains are not mountains w/forests
Have fun,