I've been avoiding them too. A tool would be nice but it's hard to model them too precisely with sweeps especially if the radius of a bend is curved so it creates a helical curve. I've done some stuff for some monorail beams and what I ended up doing was modeling the concrete down to the blockouts at the tendon ends and just made 2d groups for the elevation and plan view of each tendon. I came up with some solutions for modeling the tendons but it wasn't worth the extra time to have them in 3d when our main concern was getting the concrete detailed right.
I think the main wish I would have in addition or before a post-tensioning tool would be the ability to create model lines than have end points that can be drug in any direction instead of just within the plane it was created on, that way in addition to creating post tension cables we could do sweeps for object with similar curves like railings and spiraled stringers, etc.