Many threads on this board have reported readability problems, and/or apparent media degradation, that finally could be traced down to the use of paper labels on DVDR discs.
In the most extreme cases discs became totally unreadable and, quite often, removing the labels fully restored the readability of the discs.
Many possible explanations have been voiced on this board.
Here is an "offfical" explanation from Memorex (excerpt from Memorex, Reference guide for optical media)
Paper labels are not recommended for DVD discs.
The expansion and contraction of moisture in the paper and the accumulation of heat in a DVD drive can alter the flatness of a disc enough that it falls out of the tilt specification and may not be able to be read.
Bottom line: unless you find a recommended source of labels causing no problems, it's strongly recommended NOT to use paper labels on your DVDR discs. Of course, if you already did so and have no problem, don't panic. But if these discs start to have reading issues in the future, you'll know what to try next
Here are some threads on this board, where the source of the problem could be traced down to the use of paper labels.
If you find more threads, let me know, I'll add them.
Feel free to post here your own experiences, good or bad, with paper labels. :).