Fractals are often delicate, and can very easilly be ruined by postwork. On the other hand, effects such as "levels" in Photoshop or "Clarify" in paintshop rarelly detracts from an image. Postwork can be used to add textures and layer several renders ontop of eachother. Whenever working with Ultra Fractal I tend to render in PSD format so that I can alter the hue/saturation/value and opancy in photoshop for each layer in postproduction.