The simple answer - Lean the engine any time that it is operating at less than 75% power.
Use the POH to determine what your chosen power setting is in % power and base the decision on that.
The POH recomends leaning in the climb when above 5000ft because leaning is necessary to produce maximum power above 5000ft and also even when leaned at that level the engine is not capable of producing more than 75% power at full throttle.
So simply - in the climb lean when above 5000ft, in the cruise lean at any level unless you are using 75% or more power.
Regards,
DFC