Like Ted, i reverse the two hive bodies in the spring, usually early May
after they've been flying for a while and the nights are not so cold.  This
goes along with cleaning off the bottom boards, which are usually covered
with debris/mold/mildew/dead bees.  I place a terramycin patty in between
the brood boxes, and treat with apistan.
 
Some years, many colonies will have expanded their brood nest down into the
lower box already.  I reverse them anyway, to encourage them to clean up
and utilize all of the combs.  Then sometimes i add a third chamber of
combs to the strongest colonies.
 
Yesterday i found several colonies prior to reversing which were building
queen cell cups.  They were totally ignoring the combs in the bottom box.
Had i not reversed the chambers those queen cells would no doubt have had
eggs in them soon, as the bees were getting crowded in the upper brood
boxes.
 
Several years ago i read that you should reverse every 7-10 days during
swarming season.  So i did, year after year.  And i pulled combs and cut
queen cells like mad.   This was a lot of hard, hot work,  wasted tons of
time, disrupted the bees very much,  and got them quite upset.  The bees
would boil out all over and get squashed.  And yet they would continue
going about preparing to swarm, no matter how much super space i gave them
above an excluder.  Now i just reverse the one time, when spring cleaning,
medicate, and make sure they have food and space for the buildup.
 
During swarming season i do the "quickie swarm cell check" once every 7-10
days or so.  That is, just crack the brood boxes apart, tip up the upper
one, apply some smoke gently to its underside, and scan for queen
cups/cells.  If they have lots of cups i can remove a couple frames of
brood to a weak hive and replace them with empties.  Often this will do the
trick.  I realize you can't see ALL the potential queen cells, but i'm just
looking for an indication of colony status.
 
If i see actual queen cells, with eggs or small larvae in them (sometimes
its very easy to notice the presence of royal jelly without looking too
hard), i remove 3 frames capped brood from the upper box plus a comb of
food and replace them with empties, also do a quick spot check of the
remaining brood frames in the upper chamber.  The pulled combs, with queen
cells, go into weak hives or nuclei, or get combined 9 to a chamber and
form new colonies.  This has helped in keeping up with mite/winter losses.
 
 
After a few rounds of inspections, the swarm season is past, and the bees
are occupied with foraging.   Now comes the more easy-going routine of just
adding supers all around as necessary, and anticipating a bumper crop...