I suggest that units defend on 3 or less in territories with snow, because the cold will accumulate casualties.
293. Inf.division reported this losses for 16 nov to 31 dec 1941:
217 dead, 533 woundet and 99 missing in combat
806 sick and 330 frostbite.
So the division lost more of its fighting power to sickness and frostbite because of cold weather, than in actual combat. Now during the summer, the division would have lost men only to combat, and this would best be modelled in A&A games by let units defend on 2 or less in summer, and 3 or less in winter.
During winter 41/42 the Germans lost 280 000 men to frostbite, and during winter 42/43 they only lost 142 000 men to frostbite. This is because in the last winter they dug into fortified positions, and fortified positions = less frostbite. Modelling this into A&A games with inf attack at 1, and defend on 3 or less in snow. Snow and terrain favour the defender, always.
All info from http://www.dupuyinstitute.org/