Crit is useful but shouldnt be your first priority of attributes to grab.
Hard to give a good answer not knowing your level and spec, but at level 70 if you are healing +heal and +int and +spirit/mp5 are your stats to go for. For a 70 SP, +damage, +hit (to cap), +int and then +crit.
Whilst healing spells can crit (and if spec'd might give an armor boost) crits arent as benefical to a holy priest as they are to a pally. Whilst damage spells can crit, only mind blast and sw:d would receive the bigger boost to this stat (leaving swp, vt, and flay rarely affected).