From a purely economic point of view. Got involved in a simulation on this once many moons ago. Three aircraft converging. Do nothing and all three hit.... Five miles separation always seemed to require five less track miles for one, no change for the second, and five more for the third, Level changed for two were financially more punitive - not to mention negative effects on cabin service etc.
OK very artificial situation but i tried it with students in the Sim later and 99% of the time it worked, vertical is easier (lazier?) for the controller. but if you're reallly busy it isn't always a viable solution, Especially when traffic is alrady at cruising level through several sectors it doesn't always look professional. Would use it when traffic has to perform level change before exiting the sector
Remember the 1 in 60 rule.