look at the datasheet for the 163 (tell me you aren't using the original TTL...). count happens on rising edge. so duty cycle *mostly* doesn't count.
50% duty cycle is so that you can get the highest possible clock rate. minimum clock low and high (from datasheet) are the same, thus any clock duty cycle other than 50% will reduce the max frequency. In practice, it really doesn't matter. If you are running that close to the spec, you probably should move to a higher frequency chip.