While you can get excellent scans with RW media, you can often get what would look like very marginal scans, especially after multiple rewrites (or even on the first rewrite). RW media is not really intended for long term storage though, so to me how a RW disc scans is less important vs +R/-R media. While I have left data on some RWs for 1-2 years+ without problems, I would stick to write-once media for critical long-term data.
I know I didn't really answer your question, but my comments go back to what the main purposes of a quality scan is for to me, for checking stability over an extended period of time, comparing burn quality with different burners, etc. And for media that should be used mainly for shorter-term storage, I'm less interested in the quality scan than I am interested in if the disc reads fine in a simple transfer rate test.
If I had to answer the question though , I'm much less picky about a RW disc's quality scan than I am for write-once +R/-R media. Even a 'marginal' scan, which I often see with RW media, is ok by me as long as the disc is readable because I don't really intend to use RW media for critical data and/or long-term storage as I mention above. A disc with 50,000 PIFs may be a scary thought if I was hoping for the disc to last for years, but 50,000 PIFs on a RW that'll probably get reburned within a few weeks anyway would not be a big deal to me (assuming a fully readable disc and evenly distributed errors).
One thing that I do use quality scans for with RW media though, is if the disc is wearing out and getting to the point that it should probably not be used any more. Large densities of errors at lower levels are fine by me with RW, but if the errors start getting very high then I take that as a sign that the media's probably getting worn out (though a full format of the disc might be something to try at that point).