Thanks for the quick response! I will leave the setting as-is, as per your advice.
However, out of curiousity, I did check in PlexTools, and I see two relevant settings:
"Change Book Type for DVD+R"
"Change Book Type for DVD+R DL"
Only the second one is checked (DVD+R DL). Yet the disc I just burned a few minutes ago was not a DL disc, but the Book Type was changed (according to Nero). Seems odd...