I usally need to encode in several different formats/codecs my Adobe Premiere projects: they usually have lots of effects (AE Dynamic linking and Color Grading), so having to export the same projects ford DVD, Blu-Rau, H264 720p, H264 1080p, H264 1080p CBR and VBR, and so on can be a real pain.
Is it a pro viable solution to export a DNxHD master for future conversions/encodings? Whill the quality be far worse than if i'd export from the Premiere Project?
Yes, this is what a majority of editors do when they complete an edit. You create what is called a "mezzanine" file; basically a file that is used to create your various versions. Most people, myself included, just call it exporting a master file. ProRes and DNxHD are the two codecs most often used to create the master mezzanine file.