What i've fouind in training is that it is good to keep the straights to one spot.
MARKING THE SPOT IS A GOOD IDEA.

The reason I say this is because if you mark the spot nice and high (Fac/forehead level.) You will be forced to continuously hit that spot. However if you leave a bag unmarked and hit it for 2 or 3 rounds you will notice that you start to punch at shoulder height unconciously as this is the most comfortable.

Marking a spot and aiming to be accurate at this area makes you work to get in position and think.