Hey Jessica,
What exactly are your needs regarding lead synching? The Marketo bidirectional sync will continuously update leads across your instance which have records in SFDC, so continuously using the Sync Lead to SFDC flow step should not be necessary, and can in fact cause performance issues, or make you go over your SFDC API limit.. The bidirectional sync will send all field, activtiy and campaign status updates to SFDC during every sync operation, and each operation is queued 5 minutes after the previous one ends.
Ideally you should only be using the Sync Lead to SFDC flow step whenever you need a new lead to be created in SFDC, or when there are crucial updates that need to be pushed to your Salesforce instance immediately, for example a Sales contact request.