Hi,
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 for DVD, Blu-Ray, 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?

I always export & save a master of the final project in a high quality, "lossless", codec like Cineform, or DNxHD at the highest settings.
Future transcodes from the master file are theoretically not as high quality as you might get by exporting from the original project, but honestly, I have never found this to be a real problem.
It certainly can be a big help if you need to revisit for a new transcode several years later.
Instead of trying to resurrect the entire project, match it up to a compatible version of your NLE, etc., you can just load up the master file in your current software, do the export, and move along with your day.