It's debatable whether there is a single standard scanning speed on the NEC forum, because the scanning threads ask for either 5x or 8x scans.
Given that many of the NEC 3550/3551/4550/4551 drives scan at their very worst at 5x and 8x it doesn't even make sense to enforce a single scanning speed on NEC drives in my opinion - unless that speed was 1x which would be too slow for most people.
Look here and here for what I'm talking about when I say that not all scanning speeds are created equal for these NEC drives.
If 5x scanning works for you, then use that. If 8x scanning is better, then go for that. If none of those scanning speeds give reliable results, find another scanning speed that works or better yet, get a LiteOn DVD burner for scanning.