I always roll for damage. It provides the variance for every strike in the game and prevents the "number crunching" of monster damages. However, I do roll damage die at the same time as I roll to hit, thus saving time. If the enemy have multiple attacks, I will roll them all at once (damage and attack die) if it makes sense in the game. To keep the different damages and attack die from getting confused, I roll matching color damage die to matching color attack die (so I usually have two or three "sets" of dice out and ready to use behind the DM screen)