NERO has a "verification" option which confirms that the data is an exact copy. A DVD-Video can actually get away with a certain number of errors whereas software can not.
So I have three suggestions:
1) Get Nero 7,8 or 9
2) Buy better quality media (DVD9, or Dual-Layer is much more prone to errors)
3) Get a new DVD-burner; SATA if possible (read comments at Tigerdirect or NCIX)
My stack of dual-layer discs kept producing coasters until I did a lot of research and bought a new Philips DVD burner. Even though my trusty LG had burned 1000 DVD-R's without errors my Memorex DVD9's wouldn't burn at all and my Philips DVD9's had problems too. No errors since the new burner.
For some odd reason, older drives tend to have issues with newer discs. In my opinion the whole purpose of "specifications" should mean problems should be rare, but they aren't (again, mainly with dual-layer DVD).
I still kept my older LG though and burn regular DVD5. I burn a lot of discs as I said.
Summary:
"Coasters" are primarily due to sub-standard disc quality but can still be solved by burning with a newer and better (at least more compatible) burner. Sub-quality discs are still more likely to have read issues in the future. So either reburn later or use better media now.
Always use a "verification" feature when burning software and use it for Video until satisified your Media/DVD burner combo are happy together (10 discs or more).