Although I must stress,5nm is a minimum, not a target to achieve. For traffic in the cruise and speed locked I wouldn't be using the bare minimum as this means an instant loss of separation if things change eg the one in front slows due to turbulence. 8-10nm is a more realistic in trail scenario. Even so, wake vortex encounters in the cruise seem rare,or at least rarely reported, and can be easily solved with a lateral offset.