Did you know that attics are like Black Holes? Once you start storing stuff in them, it rarely comes out (with the usual exception of seasonal decorations.) Folks just end up using attics like a closet for stuff they want to REALLY put out of the way.
All I'm saying here is don't be surprised if your attic becomes a MAJOR storage place. Mine did and I DIDN'T want to put ANYTHING up there! IF you have Batt insulation that's 'level' with the floor joists at 16" on-center, go ahead and use 5/8" plywood. Leave some room between sheets for ventilation. Pre-drill and screw them down in as FEW places as possible (in case you need to move them for whatever reason.) I left about 1" between sheets in my attic.
IF you have loose-fill insulation, use the same plywood BUT you really don't want to use too much. The reason being is because, once you compress the fill, it will lose some of the insulation 'value'. Loose fill is meant to be loose. Attach it the same as I just said. Hopefully, your attic is vented w/soffit vents and a ridge vent (or gable vents.) Whatever you put up there will be affected by heat buildup. Believe me, heat can 'age' stuff w/o you even touching it!