If they're inward dents, it could be where the sidewall belts are joined and overlapped, and not a problem. All 8 tires on my 2 vehicles have these - two depressions that run from the rim out to the tread (like spokes), about 6 inches apart from each other.
Outward bulges are bad.
http://www.tirerack.com/tires/tirete....jsp?techid=32