According to the TOS using game mechanics in a way that give you an advantage over other players is against the TOS. Hence, playing a 5 box gives you a clear advantage over other players, and if blizzard decides, it can be against the TOS.

The same goes doubly for software solutions - using Keyclone, AHK, etc etc can be considered against the TOS just because some blizzard minion has a raggy day.