When writing data to a CD or DVD using Nero 6, does comparing the MD5 checksum of the written file to the checksum of the original, give as good an indication of integrity as using the Nero 6 "Verify" feature?

MD5 is extremely accurate.
http://www.irnis.net/gloss/md5-digest.shtml

Don't know what method Nero uses to be able to say whether it is equivalent or not as good but I doubt if its superior.

Like to know how to check the checksum of a disk (volume) on the PC, it's available on the Mac, though.

I know nothing about Macs but you can use a tool such as WinHex (http://www.x-ways.net/winhex/index-m.html) to do such calculations. There are provisos though since to do this accurately (and thus meaningfully) for a whole volume, it should not be in use at the time and should also be unmounted.

Some explanation at http://techrepublic.com.com/5102-22-5490996.html

Utilities such as CheckCD and Hyperhasher can do the calculations, but for whole volumes I would expect you would get errors or inconsistencies unless the volume can be locked during the calculation.

Out of interest do you have a special reason for wanting to calculate a whole volume; usually the only reason would be to compare it with a cloned image of the same.