Hammer is always useful when you are clumped every now and then for at least 10 seconds, the most common fights being:
magaera - every rampage (at least every second, depending on how quick you kill the adds
tortos - every bats
jinrokh (well, only normal mode) - every storm
Iron Qon - P1 and 4 when you are stacked
and for trash.
Prism is especially useful when you are not clumped, but need some aoe heal quickly
jinrokh hc - after every static burst (the only HC I know so far )
tortos - after every stomp
jikun - quills
durumu - maze phase (you are clumped, but you are moving -> hammer = bad)
primordius - all throughout the fight, everyone is spread out and moving
animus - after jolt
twins - in P2 when people are dodging the waves
lei shen - pretty much all the time except for the transition phases.
other than that, when in doubt, prism. reason being - it still works when people are clumped, but hammer doesn't work when they are not. and with high vengeance, it crits for up to 450k on a single player (=> roughly 200k non crit). which is a damn lot.
execution sentence is only useful when you don't give a damn about aoe heal at all and you want to buff your dps.