I have big hands, so this may not be that applicable to you. What I've found really works well for me is to slide my hand (right hand in my case) low enough that my pinky finger is actually below the bottom of the phone and kind of pointing back towards me, in effect creating a "shelf" like effect by supporting the bottom of the phone where the charging port is. My other three fingers rest along the left side of the phone. This is the perfect position for me to reach the whole keyboard (with room to spare, in my case. Again, I have unusually large hands.) When I bring the pinky up and put it along the side with the other three fingers, it completely kills my ability to reach the far left side of the keyboard and the phone feels far less stable in my hand. This is very different from the way I held my iPhone 5, but I've gotten used to it really quickly and find: 1) it is very comfortable, and 2) it feels very secure. Not saying I won't drop my phone, I just feel less likely to.
I should add that this is with a naked 6+. My case should be arriving today, so how I approach it may change again once it's protected and a bit wider, thicker, and (hopefully) grippier.
I should add that this is with a naked 6+. My case should be arriving today, so how I approach it may change again once it's protected and a bit wider, thicker, and (hopefully) grippier.