Is this potting soil or garden soil? One of the reasons I found for moderating temps is the risk of fire with potting soil (which as you know contains no real soil). And ingredients in a potting soil could very will produce fumes or toxins that could be harmful to have in the house, regardless of what they may or may not do to any plants or seedlings.

Have you considered steaming the soil to sterilize?

I'm talking a 300-400F oven. Zero risk of fire (and the soil is moist to begin with.) What fumes or toxins can potting soil ingredients produce at those temps? It's all organic or mineral stuff. I guess things could get nasty if you have rubber or plastic in that soil. Steaming large quantities of soil is VASTLY harder than baking large quantities of soil.

Actually, since the soil is moist to begin with, the temperature isn't going to get much above the boiling point of water, which is still much higher than what we are instructed to use.

This is beginning to smell like a garden myth. Lots of those around. Myths propagate virally these days, without any explanation. A says it because B said it, because C said it because ... Most garden myths have some grounding in human desires and fantasies. Not clear about this one.

What plant "toxins" come from heating soil to, say, 200F? References, please.

Are you talking about potting soil or native soil? Pasteurization calls for a low oven temperature until the interior of the soil volume reaches a temperature of 140-150 for about half an hour. The potting medium needs to be moist and the container needs to be covered. It's actually much easier to pasteurize soil than dry bake it.

I would think that the likelihood of toxic fumes being off gassed would depend upon the actual contents of the soil.

The only time that I've ever used "sterilized" potting soil is in my university days when we reused our potting medium for propagation classes several times before discarding. I've not bothered to go through the process once once since then, and haven't had any disease issues in spite of it.

Any soil. And yes, I know that the standard "recipe" for soil sterilization is 150F-ish. What I want to know is WHY. What toxic fumes can you imagine coming off of soil at higher temps, like 200F? What ingredient in soil might do that?

I routinely sterilize my seedling soil as it seems to make a big improvement in preventing damping off.

I should add that pasteurization of food is an interesting comparison for the response of organics to heat. The standard treatment for milk is 160F for 15 seconds. What happens if you use a higher temperature? Not much. Ultra-high temp (UHT) milk pasteurization takes 275F, and it tastes pretty much the same, but offers longer shelf life. No mysterious toxins come out. But cheese-makers know that UHT milk doesn't work well for making cheese.

I'll say again that if you have really weird soil, like with plastics and or rubber in it, high temperatures are probably not smart. I've heard that plastic polymer beads (hydrogels) are sometimes added to potting soil to hold water better. So the rule could have more specificity in that regard.

So, let's call it an "almost myth". It's a rule that only makes sense in certain circumstances, and is widely repeated without explanation because, well, because everyone widely repeats it without explanation. I sterilize my soil in an outdoor gas oven that really doesn't have a working thermostat. So I turn it to "on", and it just blasts away. So if you want to sterilize soil, just blast away, but don't use it immediately, and best not do it on "weird soil" that contains artificial ingredients.