Howard et al,
While it is true that a small inside bevel on a fluter or veiner (#10 or 11, London Pattern) may not serve all of the same purposes as on other gouges (sweeps # 3 - 9, London Pattern), I believe it can still be advantageous. For one thing, it is a relatively easy way to produce a satisfactory surface quality on the inside of the gouge, which is not inconsequential since the surface quality of both converging surfaces determines the quality of the cutting edge. While it certainly is possible to attain the same surface quality by resting your slip stone on the inside surface of the gouge during honing, if there are any significant irregularities of that surface, arriving at a satisfactory result can, sometimes, be quite laborious.
The primary advantage I see, though, is maintaining the same cutting geometry as your other carving gouges. One of the often overlooked upshots of the small inside bevel is that the outside bevel can be at a correspondingly more acute angle (relative to the longitudinal axis of the gouge) while still maintaining a large enough included angle to retain edge strength. This allows whatever force is applied to the gouge, whether by hand or mallet, to be transmitted along the tool at an angle closer to the desired direction of the cut. In other words, it's more efficient. By the same token, with fluters and veiners, the more acute outside bevel angle also slightly lessens the wedging effect at the sides/wings of the gouge as it passes through the wood. (Though I'm not convinced that this wedging action is entirely a negative.)
I've used both water and oil slipstones for creating and maintaining inside bevels, and think they each have advantages and disadvantages. For that reason, I guess I would recommend using whichever type fits in, best, with the rest of your sharpening regime. I've found that honing longitudinally with the water slipstones minimizes the downside of their faster wear characteristics. This, in turn, is offset by the fact that they tend to cut somewhat faster and are relatively easier to shape. However, with the use of diamond files and plates, oil slipstones can also be shaped, and their cutting surfaces kept fresh so that they are also fairly fast cutting. At present, I'm happily using oil slipstones since the balance of our sharpening regime incorporates oilstones.
Side note: If you are going to produce a small inside bevel on the sides/wings of, especially, veiners, the slipstone needs to be somewhat narrower than the width of the inside of the gouge in order to allow you to slightly tilt the stone, laterally, during honing. This is often overlooked by advocates of shaping a piece of wood, or whatever, for honing inside these gouges.
Finally, for all but the teeniest fluters/veiners, another option for creating and maintaining these small inside bevels might be a flex shaft grinder with aluminum oxide and rubberized abrasive rotary tips. These can also be shaped and freshened with diamond dressers and/or files. Probably only an economically realistic option if you have a lot of "profiled" cutting edges to maintain and sharpen, but I thought I'd mention this in the interest of inclusiveness. We use these quite regularly, though I have to admit that I still prefer slip stones for use with carving tools.
Don McConnell
Eureka Springs, AR