I think it's probably more dangerous (or at least more likely to lead to regurge) to feed them overheated f/t or f/t that have been left out too long and have begun to spoil. BP digestion is designed for fresh-killed prey---but the changes to the proteins cells that take place due to spoilage or "cooking" under a heat lamp are a lot more profound than what happens during freezer burn.

But generally? If my ball python seems really reluctant to take a suspicious food item, I try to take the hint and throw it out. Rather than pushing the issue. I'd rather spend a little extra on mice, than have to clean up a regurge later.