There are some indications that this might well be the case for some species, but not for others. I am sure that a few freezes at least "ripens" most temperate species seed. Also seed of D. uniflora, D. stenopetala and D. arcturi would also benefit from a freeze (the question being: how long a freeze?). For many other species, it would be overkill: most of the tropicals show good germination sown fresh or with a brief period of stratification in the fridge.
I generally sow the temperate seed at the tail end of winter. The seed has been through a long "dry strat". I then sow this seed on moist mix, and keep the pots as cool as possible. When the days are in the 40/50's but the nights still have frost, I place the pots outside where the natural cycles can work on the seed, in a protected spot where the rain can't reach. If there has been no previous germination, this usually does the trick, if the seed was viable to begin with.
