The answer is simple: It depends.
First, look at the specifications for your motor driver - are diodes already built in? If so, you are good. If not, adding them would be a good idea.
And about your motors - are they brush-less or brushed? Brush-less motors are a lot quieter electrically than brushed motors.
Finally - what else do you have going on? Are you using A/D converters? Are your sensors sensitive to noise? How much will some jitter on the analog inputs impact whatever it is you are trying to do?
Those are the things you need to figure out to determine if your particular application really needs a couple caps / diodes or not.