The only case where you don't have to render translucent objects in back-to-front order is where they don't overlap. The whole point of back-to-front order is so that rendering of overlapping translucent objects will work right, both in terms of the blend equation and of depth testing.
Checking that objects don't overlap might be more expensive than just doing the sort, however.
Some cases you're just not going to bother sorting every single translucent object. If, for example, you have multiple particle emitters in a scene, each emitting hundreds of particles, you're probably going to want to sort by emitter rather than by particle, and the result can look OK.