I guess I meant 'best practice' if you are using a DNG workflow - I accept that the DNG workflow is not suited for everyone. Lot's of people shoot only jpegs, some through ignorance but some because it works best for them - many press guys I know are obliged to shoot jpeg because that's what the newspaper's photo dept has specified.
I'm working 100% Raw. I was implying that for "my workflow", low volume, best practices are to convert to DNG as I ingest into LR. The speed hit doesn't bother me. At such a time that I'm ready to edit the images (picks, deletes, etc), I want DNG, full sized previews built as I very often zoom in at 100% to inspect sharpness. There's no question there is a processing and time hit for doing this (I usually do this when I'm away from the computer). So the speed hit isn't a speed hit for me. At such a time I'm ready to view the images, I want all that stuff over and done with, even if I end up tossing some of the DNGs into the trash.
Now here's a possible issue I need to test further. Lets say you start with a clean, virgin LR catalog. You import and convert to DNG, 100 images, ask for full sized previews which are (I believe) stored in the preview database, Catalog Previews.lrdata. Say its now 20mb in size. Now you delete 99 images. One would expect that the Catalog Previews.lrdata would shrink in size. It doesn't appear to, even using the optimize catalog settings. Of course, this doesn't appear to be a DNG issue only. When I imported DNGs and CR2's, looked at the Catalog Previews.lrdata, deleted nearly all the images from the catalog and optimized it, the Catalog Previews.lrdata didn't shrink.