Best practice for adding a new item in client

I have a list that the client fetches from the server. The client can add more items, and it's 'auto save' so no need to press a button. My question is how to manage the new item id (can be several). Do I need to immediately go to the server to get a new key? or can I allocate some key Ids (say 0-10) and later in the auto save call the server will know to assign new keys and notify the client?

Thank you