In truth, I run rims on one of our off-road bikes with very large dents in them. As long as the rim is straight, you can test this by holding a screwdriver against the outer edge of the rim while spinning the wheel, it isn't a major issue.
Be sure and check the rim for any cracks. If you see a crack, replace the rim immediately. Dented rims are only a problem if you can feel them when riding.
Both my rims are slightly bent. However, I don't notice anything while riding my bike. Is there any real reason to try to straighten them? Should I ignore it?