I find it easier to hit crosscourt in rallies, and not try to change the direction of the ball. When I do hit the backhand up-the-line is when my opponent has hit down-the-line with his forehand to my backhand. I tend to mix it up though and will a majority of his up-the-line shots crosscourt to his backhand and put him on the run to his weaker side but then when he starts leaning that way will go back up the line without changing direction on the ball. It's a difficult shot to hit up-the-line over the high part of net so that if you are trying to change the direction on the ball it becomes even more difficult so unless it's a pretty slow ball I won't change direction on an incoming crosscourt shot and hit the backhand up-the-line.
You should practice hitting backhands up the line both on balls changing directions and balls coming straight up the line to see which is easier for you. I try to stay a foot or two inside the sideline and will flatten this out when going for winners and hit maybe a foot above the net like on passing shots. Too far from the line and hit too slowly or with too much spin and your opponent will be able to get to it and put you on the defensive with their forehand going to the open crosscourt.