Editing midi in piano roll - mousing things left and right - I will often go right to the velocity window underneath to do a quick swipe - however - I more often than not have to DE-Select a single note to affect multiple notes (events).  Is there a hot key that de-selects... ?

Aha   ---control shift a

or the numPad 5

Or left-click in an empty area of the notes pane, assuming you're using the Smart Tool.

thedukewestern
Aha   ---control shift a

Ctrl + Shift + a is actually Select None. This clears all selections.
 
This isn't the same as deselecting one note from an existing selection. To do that, just use Ctrl.

ive become so used to using lasso, im like a cowboy !
I left click/lasso an open area (to de select all) what Dave described
but as Jonesey Jones pointed out above, that de selects everything.
 
But to de select only that one being/note, you need to hold control (using the smart az tool)