Perma-free should only be permanent if it's working for you. If you're seeing increased sales as a result of it, then keep it perma-free. If you're not, or you're seeing reduced sales (meaning the increased sales of the other books is not enough to make up for the lack of income from the free book), then go back to paid.
I jut put my "perma-free" book back to paid the other day. (Oops! I better change my KB sig line.) I actually had no intention of it ever being "perma-free" -- from the start it was planned to be "long-term free". I'd originally thought it would be free for 2 weeks and then back to paid, but I shut it down after 2 weeks and am perfectly fine with that. I gave away enough copies to be satisfied for now.
Will I ever make it truly perma-free? Maybe. But not now. If I do, it won't be until there are at least 6 books out in the series. Right now there are only 2 books in the series (with the 3rd book coming next month). I did make it free in order to jumpstart reviews of the first book and hopefully sales will trickle through for the rest of the series, but I have no interest in giving it away permenantly. Several thousand copies given away is sufficient for now.
(I had a mix-up actually, and I inadvertantly included the second book at the end of the first book, so I really gave it away BOTH books for free for about a week and a half there. I'm hoping it will be a lot of good karma for when the third book comes out, but it led to my decision to end "free" earlier than planned.)