Deleting Invalid Contacts (SFDC)

Apart of our data clean up is to delete unsubscribed leads from Marketo (not SFDC) and hide these leads from the SFDC sync.  I am wanting to perform the same operation to invalid leads and contacts.  However, deleting contacts scares me.

What is the best practice for cleaning up contact records?  Should I alert sales to update the contact information for these contacts first? Should I just check the last engagement date and remove anyone who hasn't engaged in a few months?

Any advice is welcome! / If there is already a product sheet or blog post I have missed just post a link!

Thanks!

-Ande