I always make every image a 16X22 which can be printed 22X30 or 30X40. The edges of paintings often contain vital information which you DON'T want to lose by changing the aspect ratio. You can often crop and squeeze a photo without damaging the intent. A Painting? Not so much.
Also it pays to work in the same aspect ratio. As you grow in painting skills, your work begins to come alive. Sometimes you want to rip off the earlier canvas and replace it with your new masterpiece. If all paintings are the same aspect ratio (in my case 16X22)...this will be an easy task.
I generally make my paintings 180DPI at 16X22. I have worked as low as 100 dpi and as high as 240dpi. The reasons for this are:
1. Canvas is not much improved by higher dpi AND paintings (at least mine) go to canvas.
2. Higher dpi slows the computer down too much. Save the details for photos. A photo can lose 50% details and still provide ample information for a interesting painting.
Here is a gallery with both painting and photo to provide a taste of Montana scenery. http://www.strengthinperspective.com/Pictures-USA/Montana-62/Montana.html You can see that detailed images are best in straight photographic format.