I like to keep any hydraulic ram closed up when not in use so the piston rod is not exposed to the elements. I also like to keep the rods lubed with silicon, makes them slide up and down easier and protects them when they are extended.
This was my 'common sense' approach (not an expert opinion!), but your question got me to wondering, so I researched the interweb; most common thing I saw was "keep stored cylinders fully retracted".
I'd be interested in other evidence.
Lyle