84 is on the high end of what's still safe - go much higher than that and you'll start having some problems (lower hatch rates, deformities, and high mortality). Having the drop at night will help though.
The biggest problem with incubating at the mid to upper 80's is the neonate size. Eggs incubated at a "natural" temp of mid to upper 70's (the temps of the earth at nest levels in their natural habitat), will take slightly longer to hatch (for me, about 7-8 months) than those at higher temps (6 months).
However, the babies resulting from the lower temperature incubations will be often a full inch longer, and much more robust. A breeder I know usually incubates their eggs in the mid to upper 80's, and has nearly 100% hatch rates - though her babies are never more than 2- 2.5 inches total length. My babies are all about 3", which is pretty darn long for a veiled. They're able to eat 1/8 inch crickets out of the egg, in most cases.
You can sucessfully incubate eggs at those temps, but it's generally "Safer" to do it at a lower temp. Once you start getting higher than 85, you start to have decreased hatch rates and babies dying as they slit their eggs.
Basically, the warmer they are incubated at, the sooner they'll hatch, and the sooner they hatch out, the less time they have to grow, and they'll hatch out smaller and weaker.
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