Active and young sighthounds are often a little ribby. I know a man with a tremendously fit working sighthound that looked like a bag of bones, it turned out that he was feeding her too many of the animals she caught and that was just too much protein for her system. It is hard to get the right balance because when sighthounds are in the right weight range they gain and lose very easily. I do think it is worth trying for the optimum weight because an overweight dog loses a lot of the joy of living but an underweight dog is susceptible to illnesses and has less energy resources to deal with problems.I have an underweight dog - I can see Kita's ribs well and her spine is also easily felt, even visible. However, she eats food meant for active dogs, gets lots of treats and raw bones. She just doesn't gain more weight (prtially because her active life).