I always hit Aimed Shot as soon as someone yells squirrel so it hits the mob about when Heart/Gnos gets there. I then follow with a quick Steady Shot and Arcane Shot so that I get both of them in before Auto Shot goes off.
On Hydross I cast MD as soon as he starts to shift so I hit him with Aimed Shot almost the second he crosses the line. This spares the previously mentioned people that have forgotten to stop DPS.
If I cast MD in the middle of a fight like Leo I hit with Steady, Arcane and Auto just because they're faster.
If you have a tank that you know will need some extra help setting agro I always pop Bloodlust Broach at the start of the pull so my MD shots hit that much harder.