What i have on my WHM for melds (Main healer in raids):

Left Side: Acc > Pie
Right Side: Vit > Acc (Cant meld pie on right side because of vitality melds)
And when i have enough acc and pie is capped on a piece i meld crit in every slot left. Not a huge fan of det and ss. Det is usually just a very minor buff and spellspeeds full potential can only be reached when chaincasting like a blackmage, but a healer obviously doesnt chaincast and 1-2 extra regen ticks over a span of a minute is never required.