The majority of RJ45 connectors are tab down when mounted on a pcb, especially the older/simpler connectors. So I'd expect to see tab down on most equipment, and it was certainly the norm a few years back - the main reason that you might find an inverted connector would be because the pcb was mounted face down within the case (for whatever reason).
Newer connectors with integrated LEDs seem to come in both denominations. I have no idea why that is though. Maybe the LEDS-on-top-are-easier-to-see theory is a factor. I also wonder whether the switch from ISA to PCI might have also been a factor (they are opposite in orientation), Who knows?