The honey flow is starting in the ECWBA area. Today in addition to numerous bushes in bloom, I saw raspberries, clover and black locust in bloom. In some areas the black locust honey flow can be significant.
Overwintered hives have already began storing nectar in the honey supers. This year's new package hives have not yet reached a population large enough to generate a surplus for the honey supers.
Overwintered hives, of medium to strong strength, should definitely have a honey super installed at this point. Check the supers weekly and be ready to add another super when the first super is 60% filled. At this time you do not want the bees to begin storing nectar in the brood nest area if the honey super gets filled. This will cut down on the places for the queen to lay and may promote an urge to swarm.
Some beekeepers recommend what is called "bottom supering". In this technique the new empty honey super is installed underneath the partially filled first honey super. The claim is that this method motivates the bees to gather more nectar. "Bottom supering" requires more work of the beekeeper because he/she must remove the heavy partially super to install the empty super underneath. I have seen no scientifically gathered data to support this claim. Like most things involved with beekeeping there are varied opinions about "bottom supering".