I have very little experience with incise carving so it would be wise to consider this post as entertainment rather than education. I use a very small veiner. It can be very difficult to maintain a consistent depth and therefore width, particularly in heavily figured wood, with a parting tool. A Veiner has straight sides so slight variations in depth do not translate into variations in width. When a slight variation in width is required, I follow up with a flat chisel and gently widen the side of the cut. I also take the flat chisel and just shave the corners of the incised line to remove the perpendicular angle from stock surface to incise cut to soften the transition. I DO use a parting tool for some elements, cross hatching as an example.