you should update your pbn once in awhile and also dont link them all using the same anchor texts. you need to keep track of each anchor text you are using in each PBN so you know how much percentage of exact anchor text you are using for all your backlinks. That's very important. Also, if i were you, do not use spun content (even if you fix the grammar later) for PBNs. and like everybody said, you need to diversify where you link to. do not just link all your PBNs to your money site, sometimes you need to link to your web 2.0 or other tier 1 links.
i think the most important thing is ALWAYS to keep track all of your link structure. You need to know your own link diversity and your own anchor text diversity.