Lower centre, with Current target and focus target frames (usually tanks) above grid as a way of focusing 2 targets.
Almost all my heals are mouseover macros that default back to current target when no mouseover is found, and target focus (with priority) with a modifier.
ElvUI is good for this - has two modes;
/DPS - puts the frames to the left hand side
/heal - puts em in the lower middle (and auto adjusts from group up to full 40 man TB raids)