Our Spider spends each winter dry and snug in the garage on a SORN with an Optimate on the battery permanently, that's what they're made for.
We had a shock last spring however when the clutch went all floppy as soon as we tried to drive it out of the garage. The master cylinder was knackered and had to be replaced.
I reckon sitting for a long time gives old hydraulic seals the chance to stick, then rip and break when they're next used, so this winter I have operated the clutch and brake pedals on a regular basis.
Running the engine for short periods gets oil round the engine and keeps seals from sticking, but lets acids condense in the cold exhaust system so its a debatable advantage. However a few minutes of running the engine probably won't generate too much acid and that's what we've gone for this year.
I put small blick coloured spots on the wheel rims and rotated the wheels every month or so.