In any type of heater, if the voltage tolerance is on the lower side with respect to the voltage within which the heater was actually designed then there is no problem.
But, for higher voltage tolerances it depends on the watt density on the basis of which the heater is designed.
If the heater is designed with maximum permissible watt density, then no voltage fluctuation tolerance is possible and on the other hand, if the watt density is lesser than the maximum permissible watt density, then 5-10% higher side voltage is fine.