My theory is that if you are mining in the winter you should discount power costs completely, since the heat generated by your miner will contribute to the heating of your home and thus cause your heating system to run less. Is this valid? The profitability calculators don't seem to take this into account.
This would be approximately true, if:
Your regular heating system is a resistive electrical heater. (Not gas or oil, not a heat pump.)
Your mining rig produces no more heat than is needed to heat your house at any given time. (This may be false for people with lots of hardware, or living in warm climates. Note also that your heating needs may be much less some times of day; for instance, you probably turn down your heat when you're away from home or asleep.)
Your heating system is controlled by a thermostat in your own living space (some people in multi-unit buildings have a single thermostat for the building, or some open-loop system)
You normally pay for the electricity used by your heating system (rather than having it included in rent, for instance) and at the same price as domestic electricity use
The heat from your mining rig is circulated through your house just as efficiently as your heating system does it.
I suspect that users for whom all of these are true are in the minority. But in general, it is true that the electricity cost can be offset to some extent by reduced heating costs; to what extent depends on each individual's situation, so it's not clear how a general-purpose calculator should account for it. Therefore, they don't.
Note that by the same token, running your mining rig in the summer may incur additional cooling costs. Mining calculators typically don't consider that, either.