Using nofollow might not be worth in order not to send out link juice (in other words Google Page Rank) in this case.
But linking to those sites you are DEFINITELY giving out your link juice! Doesn't matter what Matt Cutts says now in his blog, you can test for it on your own.
Remove totally those links and your Google PR will go up (you have to wait some months). Then put them back in and you will see your Google PR going down again.
These types of techniques anyway might be somthing called "Black-Hat SEO technique" or let's say at least "Grey SEO technique". There is a discussion about this on Google's chit chat.