When CD writers first became affordable to the great unwashed (yeah, that’s me ) I used to do on the fly copying all the time with no quality problems; the only problem that did crop up occasionally was making coasters because the drive couldn’t read the source disk fast enough to feed the writing software (normally due to a dirty/scratched source disk). This was in the days before burn-proof systems so if the writer ran out of data the write failed.
I would guess that it is the same with DVDs, but don’t quote me
DVDs and CDs have a built in error correction strategy, so the occasional bit that is mis-read shouldn’t affect the whole.
Having said that, my gut instinct is always to copy to the hard-drive first; I don’t know what effect pausing and re-starting a write (i.e. actually making use of the burn-proof feature) has on the quality of a DVD, but I’m thinking it’s got to be worse than a straight one-time write.