Hi Alan,
If you are aiming to keep those leads out of SFDC until they reach a certain score threshold or meet other criteria, then the suggested practice would be to add a rule to the sync that prevent it is the lead score (or whatever crtieria) is not met.
Then, instead of putting syncs in all your socring campaigns, develop a single campaign that syncs the lead to SFDC when the criteria is met.
Here's an example using a lead score of 50 to qualify someone for sync to SFDC
"New Lead" Campaign
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Smart List - "Trigger - Lead is Created"
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Flow Step - If Lead Score is greater than 49 then "Sync to SFDC", default choice is "Do Nothing"
"Warm Lead" Campaign
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Smart List - "Trigger - Data Value Change" - Lead Score is greater than 49.
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Flow Step - Data Value Change - Lead Rating is "Warm"
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Flow Step - Sync Lead to SFDC
Again, this is pretty simplified, but hopefully you get the idea. If you use "Sync to SFDC" anywhere else in your campaigns, then just be sure to form it like the Flow Step in the "New Lead" campaign, unless, of course, you want all leads that go through that campaign to sync, regardless.
Again, hope this is helpful.
John