Well, high spikes are not good but temps as low as 90 will do absolutely nothing to start the growth process. Try to get to 98-99 to get things started the first few days. Don't open the box the first few days. Chickens don't get off the nest for the first few days, so it is important not to have fluctuations and maintain that critical temp the first few days. After, if you have spikes that get to 102, the eggs should still be okay. Even if the temp drops to 97 they should still be okay. That's the range you have to work with. Your going to have to open it to turn the eggs everyday (or tilt the box so they will roll), so brief swings in temperature shouldn't hurt them.
I'm editing to add that by "okay" I don't mean you can keep it at high or low temps, I just mean that the chicks shouldn't be harmed by brief fluctuations in temperature. If you find your temps to high or low, adjust them.
