I've never had an NPC assassinate miss when I was at full health. NPCs seem to use the old assassinate code where you could not even try if they were hurt at all. I usually take off a piece of HP gear, wait for the tick wear it again and head in. No problems.

On a side note, the IMMs implemented code before assassinate was changed to how it is now so that if you cast a damage spell on yourself, you lose some practice %s in that spell. That's because APs were always lightning bolting themselves to avoid being assassinated.

PCs cannot backstab you if you're below a certain % HP (not sure of the exact number). I don't know if backstabbing NPCs have the same limitation, but they may.