Dave.
IMHO it depends on the topic (ie keywords) and the geographic area that you are advertising to.
For example, I am very careful with the Search Network in AsiaPac (Thailand seems to be particularly dodgy for my topics for some reason) though with lots of negative keywords things are a little better.
There are some egregious paid-to-surf sites that you want to avoid one way or another in any case...
Rgds
Damon