I've not tried looking at the hole during my actual swing. I do look at the hole a few times as I take practice putts but I tend to have poor contact if I do not watch the ball during my actual putt.
On the practice green I do focus on speed over line. Let's face it, whatever break you see on the practice green has no impact on the breaks you will see on the course. Ideally the practice green is prepped to be similar speed to the actual greens so that speed control can be taken onto the course.
I try to hit several putts with similar back swings on different slopes...Flat, Up-hill, Down-Hill and Side-hill, to get a feel for the speed. I still need to adjust for the actual steepness of the slope on the green during play but this at least gives some frame of reference.