I usually don't bribe the Roman armies, sometimes I do though depends on the situation.
I normally prefere battleing them , for experience and just for fun.
ofcourse when I see an army I just can't defeat with my already worn out army, I will try and bribe .

and I agree that bribing an army on grounds that it will reduce their strenght is not a good way especially in the later game ( post marian as Romans ) since every other Roman factions when playing the Julii have so much money that they can recruit every army that you bribe without even looking twice at what their spending..it's much better to try and sabotage the enemy army buildings if you really wan't to reduce their military strengh.