I am experiencing the same problem.
Nero BackItUp 126.96.36.199 (from Nero 188.8.131.52a) always selects the LOWEST available write speed.
Example: 2x selected on NEC ND-3500AG 2.TB with Verbatim DLP DVD-R 8x (2x, 4x, 6x, 8x, 12x available) - This has the unexpected side effect of creating a coaster.
Example: 4x selected on Plextor PX-712A 1.06 with Verbatim DLP DVD+R 8x (4x, 6x, 8x, 12x available).
I am 100% certain that an earlier version of Nero BackItUp automatically selected the HIGHEST available write speed, I'm just not sure which version.
I haven't tried downgrading Nero, yet.