When I first started creating DVD archives the best advice for labelling them was don't! This was especially true of for CDs, where one of the layers of plastic can be very thin, and easily affected by some inks. The worst thing to do, which some did, was to stick labels on the disk!
Of course things may have changed a bit since then but I have never written on my archived image disks. Instead I use pockets (http://www.secol.co.uk/DS.SHTML
) to store them in that have space for a paper label. I don't know if these ones are available outside of the UK but I'm sure that something similar must exist in other countries. I don't use the supplied labels, instead printing my own. It's certainly more expensive and time consuming than writing on the disc but then these are archival quality pockets. I have been known to mis-label a disc, but being paper based this is less of an issue than having written the wrong details in ink!
I have labelled some CDs that I made for use in my car. These were printable discs and I used a TDK pen specifically for labelling CD/DVDs. The writing has bled out sideways onto the disk although it hasn't, as far as I know, affected the stored content. Therefore if you are going to write on a disk make sure you use the correct pen for the correct disk.