Good points made by Jeff and Cartman about the different cams. Harsher cams by nature store more energy in the bow and usually require a stiffer shaft than a softer cam for any given draw weight. That's one of the things that cause variances in spine requirements.
Answering your other question, YES, you could possibly drop to a lighter weight tip if the arrow shows being borderline underspined. It's just not an option I use because it hits the pocket book. With a lot of today's broadheads getting to cost $30 or more for three I think it's so much easier and cheaper to sometimes back the bow down a few pounds and live with it. Especially if shooting 70#.
I know you've heard or read this so many times, but I will repeat. There is nothing walking this continent that 60# won't kill so if you're shooting more than this it's just extra and often unneeded energy. All it does is make the shooter work harder.