I abuse my seedlings as much as possible. Last year, I did lose quite a few early ones from letting them get down into the high to mid 30s at night. They didn't freeze, but the plants turned yellow and quit growing. I think I must have killed off the beneficial bacteria in the media. This year, I am going to move my minimum temp up to at least 50 and see if I still lose any.
My day time temps are very high, too. Heat doesn't really kill plants; they die from transpiring all their water too quickly and drying out, which obviously is exacerbated by high temps. But as long as they don't dry out, seedlings can live through very hot days.
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