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Only Google really knows the answer to that question.

Some people will say yes, and some will say no, but for me personally I'd go ahead and disavow them as I think nofollow links help at least to some extent and in some capacity.

It's hard to say from a SPAM score perspective, the best thing to do is base it on your thoughts as and when you see them, but certainly the low quality ones will more than likely want to be disavowed. Look at anchors, if they're generic or brand then I wouldn't worry about them, but any SPAMMY ones using money terms I'd disavow immediately.

I hope that makes sense. If you let me know your URL I wouldn't mind taking a look for you.

Yes, it will be a bit of a pain but you need to take the time to disavow them all. I had a similar problem with one of my websites last year and it took me a good few hours to deal with successfully.

I created a handy disavow file generator over at Lead Peep if that's any help to you. Just paste all of your links into the text field and it'll give you a file to download ready to upload straight to Google. You can find it here.

Good luck - let me know if you need any help!