I have been importing my D800 NEF files as lossless compressed DNGs into LR4. I decided to check on the effects of using the lossy compression, so I created, using the DNG converter 7.2, two DNGs from a single NEF and imported them into LR's catalog. Using the Compare feature, I looked at both files at 100%, and I cannot see ANY difference between the two. The lossy files are less than half the size of the lossless ones, and they open more quickly.
I usually use LR4.2 to do my NEF->DNG conversion, but Adobe "forgot" to put a checkbox in the preferences to allow lossy compression on import. However, you can import them as lossless DNGs, then use the "convert photo to DNG" utility in the Library menu. Here you can opt for lossy compression. It will also allow you to delete the uncompressed file upon successful conversion. All your LR edits remain intact, and the catalog is unaffected.
Yesterday I converted about 55GB of lossless DNG files to the lossy variety using the LR utility mentioned above. It takes about 7.4 seconds per image on my HP laptop with a quad-core i7 proc and 8GB of RAM. I freed up about 30GB of HDD space (55GB->25GB).
Purists might argue that I'm throwing away some data, and I suppose that is true. However, if there is no discernible difference (that I can see), why not save space and improve speed? Your approach might be different, but this works for me.