The general goal of IPC is to allow users to easily switch their manufacturers. So very high on their list is to improve portability and reduction of manufacturer dependent variables.
Reasons i remember as stated in IPC:
- Rounded corners in stencils improve manufacturability (less process dependency as you define the radius not the tool used).
- Also for stencils rounded corners improve how well the paste is released from the stencil (I did not fully understand the physics behind this but i assume a lot of testing was involved here).
- Rounded copper improves the surface tension behavior and therefore flow behavior
- IPC never ever suggested the use of pure rectangles and only suggested the use of oval because of a lack of support in the more popular design tools. (They now expect every tool to support rounded rectangle which is why they suggest it for future use)