We stopped parking any unused domains we have about a year ago. For those domains we find that it works better, and generates more revs, if you simply put up a WP site, add adsense, grab a few articles from ezinearticles, or similar free article site and let it sit. Basically this way you are cutting out the middleman, ie the parking company, which mostly use google for the parking ads anyway.
Since parking is based on natural traffic, type-in, if you have a bunch of domains, then setup one WP and point a few of those domains to that one.
just a thought...
