J.Y.
Hi C.,
I don't know anything about Rohnert Park except the area that is very built up with shopping. I suspect there are some nice areas but I don't know about them. As for Santa Rosa, Larkfield/Wikiup seems to be a well approved area but it is the farthest north in Santa Rosa right near Windsor. Windsor is a lovely town. They have designed the "town green" with all sorts of wonderful activities including farmers market, summer movies under the stars, live music and festivals. I live in Windsor and can't say enough good about it. In Santa Rosa, I also hear Bennett Valley, Rincon Valley and Hidden Valley areas are nice. The McDonald area is near downtown and has some lovely old mansions and some cute little houses too and you can walk to downtown activities if you are into that sort of thing.
I know you asked what to avoid but I have given you some good neighborhoods to look into instead. I don't know the names of the bad areas except the obvious Apple Valley, parts of downtown, near Coddingtown Mall, parts of southern Santa Rosa I believe would be wise to avoid.
Good luck. It really is a pleasant area to live.