Crystals in the resin will fall to the bottom, and suddenly you are left with a lot less resin than hardener when you need it the most.. If you heat resin to get rid of the crystals, make sure you check that there is not a layer of chrystals left at the bottom.

Most trouble with epoxy are actually caused by not stirring long enough before applying. Mixing it cold is not a problem if you heat it enough to start the reaction once applied. But it have to be stirred well, which is even harder if it's cold.