Ok, so, here is how I understand it.
Google knows if domains are parked, and don't index, and, may be, don't pay any attention to those pages whatsoever (at least it would make sense). Here is a link to Matt Cutts video: https://www.youtube.com/watch?v=eF8i6rKojXQ
So, there is a good chance that those parked websites do not have any influence in any way. However, if, for whatever reason, google doesn't recognize some of them as parked (let's say 1%, which will equal to 200+ websites), and "assigns" duplicate content "tag" to those, you might get hit by correlation. Somewhat "friend of my enemy is my enemy" system. Therefore, if unique content is not possible, nofollowing all those links might be not too bad of an idea.
I would recommend sample testing. Unfollow links on large enough amount of those domains, see how it affects rankings etc., in a month or so, put those links back to follow, wait, see how it affects, then do the same with another sample. This will allow to figure it out pretty accurately within couple months for sure.
If you do do that, write a case study, post it here, it will be interesting case to look at 
Cheers:)