Here's my opinion based on looking at about 1200 DVD quality scans from BenQ and LG 16x DVD writers. The quality of the burn (as measured by total/maximum correctable error levels) does go down when you exceed a certain speed, however that speed is typically rated speed+4x and the quality doesn't noticeably (if at all) improve at lower than rated speeds.
I've seen plenty of underrated media.
The most notable example is 2.4x-rated CMCMAGR01 that burns rather poorly in BenQ DW1620 at 2.4x (2500+ total PIF) but shows a marked improvement (to 700-1000 total PIF) at 8x.
Some 4x media like RICOHJPNR01, TYG01 and YUDEN000T01 can be reliably burned at 12x in BenQ DW1620. I burn them at 8x because I am trying to be cautious, and they give me excellent results every time. Burning these media at below rated speed would be a waste of time... you can't improve a burn much past 100-300 total PIFs.
The only example of (slightly) overrated media I've encountered was early 16x-rated MCC004 that tends to have somewhat lower errors when burned at 12x.
With good brands (Verbatim, Fujifilm, TDK, Philips, Maxell come to mind) burning at rated speeds is very safe and I've seen no need to slow down.