Converting to DNG immediately costs only time, but can pay off in other ways. I work for a photographer who shoots high volumes, and though a single shoot can take 5+ hours to ingest and convert to DNG, the conversion time is completely worth it to our workflow for several reasons:
1) The photographer can make her edit based on a properly rendered proof created based on the ACR defaults I will start with when adjusting. (you mentioned this one)
2) Images that are good enough to be shown to the client in low-res/web proof form without further ACR adjustment can be converted out based on the full size JPG preview built into the DNG
3) Our shoots often end late in the day. Though I'm not there to start working on the images overnight, the computer is. Starting the DNG conversion as soon as the images come back from the shoot actually _saves_ time later. DNGs are obviously much faster to convert to web proofs for a client than original camera raw files because of the built in jpg preview. Since the time-consuming DNG conversion was done on the front end, I can very quickly batch out the proofs and upload them almost immediately after finishing the raw adjustment work.
I should add that using Mark Rochkind's ImageIngester Pro (http://basepath.com/ImageIngester/
) makes putting the DNG conversion step on the front end of your workflow completely painless: just fill in the right bulk metadata fields, attach the hard drive containing the shoot, and go. Great program.