I would never use snap caps for season storage only for dry firing practice. THIN brass and nickel plated ones with hard black rubber primers are the best. Set springs in quality SxS are myth just like ringing barrels to test solder joints between ribs and tubes.
PS. If spring tension bothers some of you I suggest Beesley actioned piece not from Purdey (insanely expensive). Even when cocked main springs are actually not under tension. If I ever buy another SxS that is exactly what I will get.