Double butting is worth it particularly for front wheels.
They're
more worth it for rear wheels - respected opinion is that the increased elasticity of the narrower butted section allows for a little more movement, reducing flex at the elbow, which is what generally causes breakages. Jobst Brandt has a lot to say about it on rec.bicycles.tech if anybody fancies a google.
Having said that some builders put plain gauge (or heavier butted spokes) on the rear drive side to (in their opinion) counter the effect of the difference in tensions either side of a dished wheel. I don't, personally, but Chris Juden, amongst others, argues that it's a good idea.
Double butted spokes are worth it at both ends IMHO.
