DAU contacted the question submitter and determined the context of the questions is under FAR Part 15 - Contracting by Negotiation.
"Should NEGOTIATIONS be performed for all sole source awards?" Short answer is yes, because the act of using FAR Part 15 is a series of negotiations.
If I change the question to "Should DISCUSSIONS be performed for all sole source awards?" The short answer changes to no, because neither FAR nor DFARS mandate Discussions for award of a sole source contract. Be sure to check your agency’s specific policies.
However, in my experience I can’t recall ever awarding a sole source action without conducting discussion because a price or other issue always arose (deals under FAR Part 15)
Best Practice: If your requirements fit, use a streamlined method like FAR Part 13 - Simplified Acquisition Procedures or FAR Part 12 - Acquisition of Commercial Items
It’s possible that, without discussions, a Contracting Officer can arrive at fair and reasonable pricing, appropriate terms and conditions, and compliance with myriad other requirements with (to name a few: FAR 15.406-3 Documenting the Negotiation; PGI 215.406-1 Prenegotiation Objectives; PGI 215.406-2 Certificate of Current Cost or Pricing Data; pricing topics in FAR 15.402-406; DFARS 215.371-3 Fair and Reasonable Price and the Requirement for Additional Cost or Pricing Data).
If we had a team of academics study this problem by analyzing actual historical contract files, I predict they’d find a close correlation between complexity of a requirement and likelihood of achieving a sole source award without discussions. Sounds pretty logical, right?
Remember that “Discussions” are not synonymous with “Exchanges” so you can find yourself conducting multiple Exchanges yet then conduct no Discussions (see FAR 15.201 Exchanges with Industry Before Receipt of Proposals; & FAR 15.306 Exchanges with Offerors After Receipt of Proposals). This happens all the time in competitive actions were the Government signals preference to award without discussions.
Finally, whether or not you conduct Discussions, follow your agency's guidance for documenting the negotiation in a Price Negotiation Memorandum.