The only problem I would see is if would they have enough space to push the top off of the egg. I think the large hatcheries have them spaced pretty close and the chicks end up sitting on top of the other eggs. Do you have yours in an upright position with the air cell on top?
I would rather...
Just don't give up on them if they don't hatch on day 21 (or 28 if they are guineas, turkeys etc.) Our last power outage took my incubator temp down to the 70's before we got the generator hooked up. I still had a decent hatch, (some on day 23) although a few didn't make it.
Too cool a temperature may delay the hatch a little but it hasn't ruined the eggs. During power outages I've had my temp drop lower than that and the eggs still hatch.