Hi Peg,
It depends on how your sync is set up. If you have a bi-directional sync, than the leads will automatically be synced when they are imported. You will know if you have a bi-directional sync if you have a campaign in your instance that says 'If a lead is created, then sync to SFDC.' If you do not have a bi-directional sync, you will be able to import the leads and they will not be sycned.
If you do have a bi-directional sync you can turn it off and manually sync leads moving forward, but you will be on the hook for setting that up. If you go down that road, I would recommend creating a plan to determine which leads will be synced, when they will be sycned, and who at your organization is in charge of managing the sync.
Hope this helps!