The metal direction is mainly defined to help the std cell routing by the PnR tool.
You could route in the opposite direction, but the PnR router will be disturbe.
if your design does not respect the direction over the std cell, and the std cell density is "high", you could have some issue during the routing phase.
you could unrespect the direction over the macros, because normaly the density of nets which will cross over the macro is low.
During the floorplan phase, some time, I did a prototype placement+routing to "check" quickly the phasability.