I am proposing that a checkbox be provided at the login prompt for a newly installed client that allows you to specify if you want it to synchronize.
I use Inkdrop both at home and at work, but I do not synchronize at work because I do not want my personal data accessible on a work system, nor do I want work data to be stored outside of their network.
To accomplish this now, I simply turn off synchronization on my work PC.
However, the first time I logged in it immediately tried to synchronize my personal data to the cloud.
Once sync has been disabled after first login it appears to stay that way after a logout/login. It might only apply to first time login from a client.