I believe Julia refers to already existing keys in CommonData table.
No - it is not safe to delete them. A module developer created these CommonData pairs of key/value to be used most likely as a select field. He/she may have hard coded them and if you remove it the module may simply crash.
To be 100% sure you would need to inspect the module's code and also make sure that the value to be deleted has not been assigned to any record yet. To complicate it a little bit more: the same CommonData list can be used by more than one module.
In any case it is generally safer to change the value (description) but not the key itself, because the key is stored in the other table. For example /Premium_Project_Status uses the following keys/values:
0 - Planned
1 - Approved
2 - Canceled
...etc.
You could change the value "Planned" to something else and it will probably work. But EPESI was designed to allow translation of any string into any other language/string. Use Admin/Language & Translation tool to translate the required string. Your custom translation will overwrite the default string. For example you can translate/rename a sting "Contacts" to "Clients" or "Customers". This applies to CommonData values/strings as well. You can simply rename existing "predefined" keys to whatever you want.