I don't recall reading anything about reflection issues with ethernet or usb cables. Makes sense ethernet cables should be shortest possible to minimize rfi, although can still get in through connectors. For usb, seems 1.5m is pretty much standard length used, I've never deviated from this length.
As far as ethernet vs cable provider coax, I've found moving cable modem close to system via longer coax/shorter ethernet far preferable to shorter coax/longer ethernet. Another advantage is one can afford much better quality ethernet cable, 1 or 1.5m AQ Vodka vs aprox. 18m price, forget about it.