I do c-prints, and the c-printer seems to do upscaling quite good by itself, but I rather don't go lower than say 150 ppi, then you start to see that the image is pixelated if you go close, and I don't like that. Then I prefer the film grain look much more, but adding film grain to a digital picture does not feel right to me, I don't want to trick anyone into believing that my digital pictures were made on film. Upscalers can make a cartoonish look which I don't like either. However, there are many options and variations, so perhaps there is settings that I would like to use. I've just recently started looking into this software.
My solution so far is simply not to go below 150 ppi ever, and preferably be in the 200 - 300 ppi range for large pictures and 300 - 400 for small (the c-printer can do 400 ppi max). In still life photo I sometimes use a panorama head to make a high res mosaic to exceed my cameras resolution and then be able to print at a higher resolution.