Sometimes it is and sometimes it isn't. It depends on the media, the burner and the burner's firmware.
Example: Verbatim DataLifePlus DVD+R 16x burnt at 6x on my Plextor PX-712A (fw 1.06) is excellent, but not much better than when burnt @12x (max).
Example: Verbatim DataLifePlus DVD-R 8x burnt at 2x on my NEC ND-3500AG (fw 2.TB) is a COASTER - totally unreadable past the first few hundred megabytes. The same media burnt at 6x, 8x or 12x on the same burner is excellent.