My recommendation would be that every record in your CRM (Salesforce) should be synced with Marketo (both leads and contacts).
If you do not do this, then you have the problem of duplicates. If you create a record in Marketo and have not synced the person from Salesforce, you will have two of that record which will be hard to merge.
From my experience you should technically always have more in Marketo as that holds all information, good or bad.
I would agree that it's generally best to have everything in Marketo. You can easily create a few base lists to use for your email marketing efforts and for targeting but it's more of an IT challenge to try and get that data synched in after the fact.