Although, i don't label my DVDs (they are backups, so do not need to look nice), i don't really believe this. Is it just me, or label glue/sharpie eating through 0.6mm of polycarbonate substrate and bonding layer on DVD is a little far fetch theory.
If they were CDs where there is only only 1 polycabornate on the recording side leaving label side protected by thin acrylic, i would be worried. IF i need to, i would not mind putting a label on my DVDs. However, i'll be only using the thinnest grossy labels i can find. Just to make sure that it does not cause unbalance duriing spinning.