I have to take cosine error into account when photographing paintings in museums. Especially with impressionist paintings, there isn't anything detailed enough in the painting to focus on! So I focus on the edge where the frame meets the canvas. If I panned to there, I would get cosine error. Instead, I side-step to the left, focus, and side-step back to center, always keeping the lens axis at 90 degrees to the painting. Seems to work well, at least for small paintings.
May the light be with you.