It does matter with the grip, because you engage and disengage certain groups and stabilizers.
Chin ups will incorporate a lot of the biceps.. If you want to work the back with pull ups, don't chase numbers. Don't do half reps, and keep the form strict. If you don't have a pull up assistance machine at your gym, you can purchase a set of bands that help. I use these now and then to get a killer session for those supported reps. You can use regular bands. I purchased this pull-up aid, its OK. Bands are cheaper, yet this is slightly better in some ways.
Grips and hand placement does matter. Don't over stress it, stick with basics, if traditional pull-ups done correctly than it's more than enough for growth and development. As you grow and advance, switch things up. Remember don't chase numbers, just do them correctly!