When using TTL compatible chips and rather high pulling resistors, then only a low strobing scheme can be used, just because the low state is so much closer to signal ground than high state to Vcc. So, with a rather high pull-up resistor, we can accept much higher injected noise, when no switch contact is closed, than we could, when strobing high and using pull-down resistors. Even, if this noise issue would be rather irrelevant, one would always strobe low with TTL compatibles, from tradition and intuition...

With ideal CMOS parts, where the trip threshold is exactly Vcc/2, where the noise margins, the source and input impedances are identical, there is no difference whether strobing high or low. But take care, CMOS chips aren't entirely symmterical in reality! 74HCMOS, for instance, has a bigger high state noise margin (to easier accept 74LSTTL and 74ALSTTL?), so that in most cases even with CMOS a low strobing is advantageous.

Kai