Cindy, thank you for your excellent question!
I have never really question the safety of peanut butter for dogs, and it is commonly used for a treat for dogs. We often recommend it to help with giving pills, or to put inside a toy for the dog to lick at and keep itself occupied.
But to be sure there are no problems with peanut butter, I did a search on the Veterinary Information Network, the online community of vets, and no, peanut butter is not considered harmful. Peanut allergies have not been recognized in dogs. Of course severe peanut allergies are widely recognized in people, but dogs do not seem to have this problem.
Certainly an allergic reaction is possible to anything, but if your dogs have shown no problems with the peanut butter up until now, I would not be concerned about allergies.
Another concern with any dietary item is that it could upset a dog's stomach (vomiting, diarrhea, gas, loose stools, loss of appetite, etc.) But if you have not seen this I would also not be concerned.
If you are using peanut butter as a treat, in small amounts, there is no problem with continuing to do so if your dogs tolerate it well. The type of peanut butter is more of a preference issue, they should be okay with any type.
Thank you and I hope this helps. Let me know if you have further questions! Dr. Marcia