Hi Xxal3naxx,
DirSync will not "create" any objects in Office 365 or the Exchange Online, it just synchronizes (copy) objects from the local AD. Therefore you do not need to upload users from CSV file to Office 365; DirSync will synchronize all 75 users from AD.
If you decide to upload 70 users by using CSV file, when you enable the DirSync it will sync users and soft match the on-premises account with the Office 365 account by using SMTP matching. SMTP matching ensures that the users that created (bulk created from CSV) in Office 365 and the corresponding users synced from local AD will not duplicate.
The detailed information for your reference: http://support.microsoft.com/kb/2641663/en-us
Thanks,
Bourne Zhang