Just a note, if you're trying to take cover or run away from another PC, that's probably interfering with their seize by force or go aggro roll, rather than acting under fire.
As to whether or not to deal harm on a successful avoiding-harm roll, remember that your job is the make Apocalypse World seem real, and keep the character's lives not boring. Is it plausible that they could avoid all harm? Would it be interesting for them to do so? Sometimes you're like "Man, there's way too many bullets whizzing around to dodge all of them. Take 1-harm from incidental fire, and give me that suffer harm roll." Sometimes you're like, to yourself, "Hmmm, I don't want them to be too knocked up by this skirmish, or they'll just run off to the Angel. Let's let them get cocky and press the assault into more interesting territory" and then you say "You dodge out of the way, and find cover from the hailstorm of lead and fire. Want to send some of that lead back to them, with interest, then?"