This is still the case as of SW2015 SP5. You have to create a proxy plane sufficiently far away from your geometry driven by variable X. Then your dimension can be equated to X+Y, where Y is the variable that goes from positive to negative.
It would be nice if dimensions could automatically flip if a negative was used, but I can understand why that would be a challenge to implement. It would require that every element being dimensioned also included a direction for reference. This is simple for planes, since it's just the plane's normal direction. But for any other element (points, lines, etc) that reference isn't so simple.