The best answers for optimal plays can usually be found in the Elitist Jerks forums, because they do a lot of testing and maths to confirm their recommendations.
On DoT refreshing, they say:
Compared to WLK, our DoT's are far stronger now. You want to do your
best to maintain very high uptime on both of them. Typically, the only
time you will hold up recasting of a DoT is if it falls off and the
corresponding Eclipse is coming up very soon--you'll wait one or two
casts and refresh at the beginning of Eclipse. If both DoT's are
coming up at around the same time when you start Eclipse, remember
that Nature's Grace will only affect the second one, so you want to
remember to use Insect Swarm first (because Moonfire/Sunfire is
stronger than IS under either Eclipse).
Cataclysm changed the way DoT refreshing works--when you refresh a
currently ticking DoT, you no longer waste a partial tick.
Significantly, if you refresh a DoT when it has only one tick
remaining, there is no loss at all. This means the optimal time to
refresh a DoT is during its last tick, thereby maintaining true 100%
uptime. Failing that, refresh at the same time you do now, as soon at
it falls off. Note: since Moonfire and Sunfire are two different
debuffs, we can't benefit from the new DoT system when replacing one
with the other. In that case you should wait for the old
Moonfire/Sunfire to fall off.
DoT ticks use your spellpower, haste, and +% damage (e.g. Eclipse)
values from the moment the DoT was cast--they do not update in real
time until the next time you cast the DoT. Crit chance of the ticks,
on the other hand, does change dynamically if you gain or lose crit
buffs while the DoT is ticking. Because the entire DoT is effected by
your stats at the moment of cast, there can be detailed decisionmaking
in when to apply them--see "Advanced Points" below for more.
From http://elitistjerks.com/f73/t110353-balance_cataclysm_4_3_dragon_soul/#DoT_Refreshing
