Technically, you never need a signed NDA. What's smart and what you may or may not feel comfortable with? That gets more complicated.
The first, important thing to note here, is that a good percentage of inventor-filed PPAs do not live up to the standards they are supposed to live up to. This means that from a legal standpoint, they are 100% useless and may as well have never been filed. From a looser standpoint, you never know they could turn out to be solid, and the ability to call your product "patent pending," in itself can discourage others from taking your idea and running with it.
But, if you filed your own provisional application and did not take the time to read Patent It Yourself and/or otherwise did not make a solid attempt to write a good provisional app - it will be healthy to proceed as if no patent application has been filed. Even if you are somewhat confident in the quality of your PPA, it still may be healthy to proceed as if no application has been filed. If you went through an attorney or are otherwise real confident in your PPA, then...
From there, there are different factors to consider. For one, protection. If you have no aspirations, patent-wise, beyond filing in the US - you can disclose your invention publicly and you will still have a year to file a meaningful patent application. Of course, note that while the country is "first to invent," Only about 1-2% of applications filed lead to interference proceedings and that the "first to file" wins about 75% of the time. And you don't want to have to deal with interference proceedings, anyway - you ideally want to be first to file. So there is some risk involved here if disclosing your invention without an NDA (and/or a good PPA), but the interference statistics also speak the the notion that there is often no race to the patent office to be concerned about....but anyways...
If you expect either yourself or a licensee to be interested in pursuing international patent protection, NDAs are a must unless you are very confident in your PPA.
And even if you have a good patent application, and are confident that a meaningful patent will issue, you will not be able to sue for infringement until your patent issues, and you may or may not be able to collect damages pursuant to when your application was published. Hence there may be situations where you want to keep your idea secret even when you have a degree of patent protection. But then, of course, in these situations sometimes you can benefit from someone taking and running with your idea....they help create a market for your invention, grab you valuable market feedback, and by time your patent issues, you can sue for infringement and/or negotiate a favorable licensing deal with the other party - you will have leverage...
And then there's the issue of...sometimes risks pay off. How much have you invested thus far? A few weeks work and $100 for a PPA? If you look at a guy like Roger Brown, he doesn't patent his inventions before licensing them. He does get NDAs signed, but do you really think that he would look forward to having to go to court and prove that a company did not already have a similar invention in the works when he approached them with his idea, or what not?
This is just to say, these companies could probably steal his ideas if they really wanted to, but they generally don't. When a company refuses to sign NDAs, its not usually because they plan on reserving the option to steal your idea. its because, well...they have a large company, and one of their employees could invent something similar without having ever come across your submission. You could be delusional, a mad inventor. They could actually already have a similar product being developed at the time you approach them with what you think a really unique idea that isn't very unique. Etcetera. More than anything else they are simply looking to avoid the potential for future litigation and liability. It doesn't mean they are not willing to license an invention from you in the right situation (although it may help for you to check up on the company's past and to see whether they have an active history of licensing new products).
All in all, NDAs, even with a PPA are smart. But they are not necessarily necessary. If something feels right, you shouldn't necessarily get caught up on whether or not an NDA has been signed.
If anyone feels that they could do a good job of summarizing what I wrote into a succinct paragraph, be my guest...
