For Nix, Winterheart is only helpful while you still have shield charges. Once those are gone, the -1 Power from Winterheart is basically equal to the +1 Burn damage you'd take without it. So feel free to switch it out once your shield runs out, unless you're really lucky with Sapphire and Diamond spawns to replenish your shield charges.
P.S. I'm totally with you, Reeler hat all the way.