Try it and see.
There is a continuum of options in between running fast on a flat track, and going up something so steep that you're basically doing 5-6 fast step-ups on each leg and then have to stop.
Steep hills will be good for sheer strength. IMHO if you do high-knee, springy drills on them it's great exercise for your calves and hip flexors too. But you won't be moving fast, and you'll know when it's no longer really 'running'.
Most people do a lot of work on shallower hills where you feel like you are running fast with a good action, but have a bit more resistance than you would on the flat, and less impact.
Vary it.