Quite some time ago I was involved in photographing some oil paintings for a collection, about 90 of them.
We found the best technique was to have the lights at 5ļ from the painting plane, which appears to be almost parallel but isnít. By checking with a light meter we positioned the lights so they were exactly equal in output at the dead centre of the paintings.
Then we moved one light back so that it was 1/10 of a stop weaker at dead centre. This gave a very slight modelling to the 3D of the paintings.
We werenít too sure whether the owner would like what we suggested, so a test was duly done and processed (EPN). One quite happy customer, so we went ahead.
Food for thought.
One other aspect was that we used a 5x7 camera with a 150 lens and a 4x5 back. When confronted with a wider or taller painting, we converted the back to 5x7, kept the 150 on the front and effectively had a wide angle lens, still with appropriate coverage. This made life extremely easy and aided efficiency enormously.