I've done a lot of painting in various homes. I get a little better every time and every time it seems I learn a new trick here and there. A couple years ago I was painting some walls in my family room...the problem was 1) we had textured walls and 2) the walls curved into eachother, therefore it was hard to decide where to stop the line and difficult to make a straight line as well.
I myself prefer to stop BEFORE the curve. I will run the painter's tape from the ceiling to the floor and do so just before the curve to gaurantee a straight line with the tape. A neighbor of mine, at that time, was an artist and told me of some stuff used on canvas to keep paints from running together. I went to my local Michaels and picked it up. It's called Gesso and I have included a photo. It doesn't take much, but all you do is dilute the Gesso with a bit of water and then put some on your brush. I would press the tape down tightly and then "paint-on" the gesso-water mixture just along the tape line where I wanted the straight line. After an hour or so I could paint my color onto the wall, remove the tape, and presto...a straight line on textured walls!
Here is a project I just finished in my daughters room. Here I painted horizonal lines and used the gesso to keep the paint from running and to keep those straight lines!