You look at it like this. Most are running 6 bolts and it's not like you have hear bunches of horror stories about the bumpers ripping off. So I am sure 6 is fine for most applications. On the other hand, drilling two holes and buying two bolts isn't a big deal, so why not do it for the extra security?

As others have said, over tightening is a bad thing. This is the reason there are torque specs for things. Stressing threads are just as bad as not tightening the bolt/nut.

My number of 70ft lbs comes from the typical torque specs of trailer hitches, not just on X's but most vehicles.