Going up in temp is easier for them than going down. Raising the temp a couple of degrees over a period of a few hours will not shock them - if anything you might see them become a tad more active and hungry. Increasing the water temp 2 to 6 degrees will increase their metabolisim.
A heater is a good idea, cuts down on temp swings which "can" be stressful. 72*F is fine, if your system/fish are doing well I see no reason to change that but as I said a heater is a good tool to keep the temp stable. IMHO gf like Moors or Bubble-eyes would benefit from a raise in temp, Ryukins or Orandas .. no biggy if they stay at 72*F, standards like shubunkins will kick into high gear at warmer temps, hungry hungry and growth rate increases.