Yep, the coldest month in the Midwest is January and already behind us. The Chinese mantid, Tenodera sinensis, has been recorded as being established in Illinois for decades (Helfer mentions it), but not, for some reason, in Wisconsin. I understand that they occur as far north as Ontario, but with this species, it is hard to tell whether a specimen that we see in the wild, say, in May, overwintered naturally or hatched from a commercially supplied ooth. The fact that since its introduction into New England at the end of the C19th it has spread steadily southward to FL, rather than northward and failed to successfully invade the desert areas along the southern border suggests that there is a limit on how much cold and low humidity it can tolerate.
Another issue related to cold weather may be the normal time of hatching. A species whose nymphs hatch early tend to have a predatory advantage over those who hatch later, but there is an increased risk that they will hatch when there are still lethal cold snaps or when small prey may be in short supply.
Normally, I would counsel on the side of caution and say keep them in the fridge, but since you have kept ooths in the garage before, and the coldest part of winter is past, I'd have to agree with Becky.
