Hi Kate, are you using SFDC? The below answer is for that scenario.
When you use the "Sync Lead to SFDC" flow action you have the option to assign the leads using assignment rules or to an individual. If you are not using your lead assignment rules, it's best to create smart lists to include the criteria that would assign the leads to the various reps.
BTW, if you use queues, the lead owner will remain blank in Marketo and the personalized emails will use the default you set up.