If your members are already have a SFDC record with an owner, the owner won't change unless you separately use the Change Owner flow action. If the member doesn't exist in SFDC, the Lead Assignment Rules in Salesforce will be used to assign the owner. You can change the assignment rule to assign the new Leads to whomever you want or you can deactivate the assignment rule (or change it to not execute if the lead is coming from Marketo) and create your own Change Owner smart campaign in Marketo (this is what we do).
Hope your implementation / integration is going well!