Machine keys are used for validation and encryption/decryption. Each site should have its machine key. They can be the same in the projects. There is no need to be different. However if you decide you could use different machine keys. As it is explained in our documentation
, to ensure that Sitefinity uses the same encryption key when running in load balancing, it is required that all web servers use the same machine key. Here there is a link which gives more explanation about machine keys.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items