For start, taxi, TO, and climb....open.
If unusually short time to warm up or open cowls drastically retard oil warm up then perhaps nearly closed during initial warm up.
Cruise, set where experience and common sense dictates.
Descent, closed until circuit height.
Circumstances and individual aircraft will dictate slight variances to the above, these principals apply through winter down to ~10 degrees-ish. The biggest variance in winter summer cowl flap operation is the warm up and taxi period, there after the open until TOC applies.
Here is a tip, might be an old Deakinism, but when you cycle the cowl flaps from open to closed (assuming cable operated) you will feel an area of light load on the lever. This is where the cowl flap wants to "fly" and is balanced between pushing against external ram pressure and internal ram/static pressure.
This spot, around half way is not a bad spot to set for cruise if you are not sure where to set, it also gives you a bench mark to a setting that is either restricting cooling or forcing extra cooling. The problem you face is the industry on the whole is not much interested in aircraft engine operation, being such a largely insignificant component to aviating, that likely you're flying a machine with a compromised single CHT probe placement. If you are blessed with an all cylinder setup hopefully you got the training to go with it, judging by your question though this appears not the case.
"Do the rear cylinders receive enough cooling in this climate if cowl flaps are closed?"
It's not that straight forward, the hottest cylinder can move from left to right bank from middle to front to rear cylinder depending on cowl design, speed of flight, angle of attack and amount of power being used. It can help to think of the airflow being more dictated by pressure differential between top and bottom deck (above/below the cylinder line) and hence flowing downward rather than front to back although it does flow front to back as well.
If you want to see a case study of IO-540 abuse in AC50 Shrikes watch the GAM pilots the country over get cowl flap operation drastically and consistently wrong.
Fuse lit, runs for cover.
Last edited by Obidiah; 27th Mar 2015 at 22:43.