What have you found the difference to be between the widths? I would think that from the outside, it wouldn't look any different. You could print 3 perimeters at .42 and get the same wall thickness as .30 at 4 perimeters, so there would be a time savings. Also overhangs might work better, since there is more support at the inside edge of the thread.
On the other hand, the thinner thread would give finer surface detail since it can turn tighter corners. I guess it would be a judgement all based on the model. If there is a lot of surface detail you would want finer threads, but go thicker if there are a lot of challenging overhangs or you are trying to print hollow. And turn down the horizontal resolution of fill with wider threads, just as you turn down the vertical resolution of fill with Fill Every X Layers.
Slic3r will adjust the flow to hit the specified thread width. I'm not too confident about Skeinforge 39, though Skeinforge 50 should be able to do it like Slic3r. I haven't had very good luke with Skeinforge and .1mm prints. Even though I have the flow rate dialed in, it always seems to pile in too much plastic with one solid layer after another. I was printing a Rainbow Dash (by request) and SF would do a slow set of perimeters of one leg, then do all four legs at speed, repeating the first leg. I could turn the flow down and keep tweaking and testing until it seems right, but I hate dialing in the settings subjectively. I like to do something I can measure to determine whether it is the proper result.
