If both views are pointed straight at the hole, then I'll look at both, in order to get a closer look at parts of the green. I always adjust the aim marker from the reverse view, though, since it is closer to the hole and allows me to make finer adjustments.
If one is crooked and one is pointed straight at the hole, then I will base almost everything off the straighter view. It is important to realize that if you have a perfectly straight, 8-inch-downhill, 15 ft putt, a straight view will read no break, but if you have a view where the hole is way off to the side -- where your grid is 30 degrees off -- the grid would claim that there is a *TON* of break, even though there is none. When your grid is not straight, uphill or downhill slope masquerades as left/right break -- either making it seem as if there is more break than there really is, or making it seem as if there is less break than there really is.
If both are crooked, that's where it gets even more complicated than normal. xD