I am definitely picky about the data, but I think most people in MusicBrainz are, or they would be using FreeDB for all their information!
Over the years, I have had a few tracks go missing - not sure why, but I'm assuming it is because fsck fixed a problem and some of my files were victims. So, I have been re-ripping my collection over the past couple months.
I will take a stack of discs (~20 at a time) and rip them, tagging them only to exact matches. Surprisingly, I had to create several releases that I assumed would be in the system. I pull from CAA and fanart first, then the rest.
My second pass is to review all the cover images for the new rips. If any of them are missing or poor quality (which is probably under ~250x250 or just bad scans) I will then scan, crop, cleanup, color balance, and export as 1000x1000 (or wider) images. It makes me happy to see the beautiful scans once they are in the system, though! I have done about 300 scans so far. I even went so far as to determine the official dimensions for the CD inserts so that I could match the proportions exactly, although I have seen some slight variations in what I am scanning.
I've tried searching for images online, but really haven't had much luck, so I gave up and do it myself.
The only tip I have is to do small batches at a time, otherwise it is overwhelming.