
It is best if the scores are alike or only about 2 points different, i.e. in a score of 14 (7 right & 7 left) would be ok but 1 right and 15 left would not because the left hip would have a score that is too high and if the right were the same the score would be 30. Think this particularly applies if the overall score is high and with one side very much worse than the other because the weight would shift to the less effected side.
Agree it is important to look at the scores back at least 3 and if possible 5 generations, also important, if you can source the information, to see the sort of scores a sire is passing to his offspring.
However careful you are you can still have a pup that will appear from a line of low scores that has a totally unacceptable high score. On the other hand continuing to select the dam and sire with great care is the only way to gradually bring the mean score for a particular breed down.
The dog with a high score, even on one hip, is carrying the potential to pass the faulty construction on and IMO more care should be taken to assess each side separately from the overall score. Of course, in breeds with small numbers this is not always possible.