Youll get many answers on this one.

i use fuel with 20% castor and never bother to much about a bit of fuel in the tank (i normaly fly till dead stick on my last tank of the day) i run my engines about once a month wheather i fly or not.

if i was storing them for an extended period say 3 months plus, i'd drop some sewing machine oil down the carby and down the glow plug hole and turn it over by hand a few times.