I've read several threads and blogs about this, but haven't seen a definitive answer for this question. When you agree to "delete" invalid leads in Marketo, I want to also delete them in SFDC so they don't sync back to Marketo. I want to clean them from SFDC because it muddies that data as well, so I don't really know if I like "Visible to Marketo". If you measure lead volume over time, those leads will get deleted, and historical reports will change as you delete leads from both databases. Is that just standard practice for marketing ops?
I always tend to delete leads far after the reporting period is over. In SFDC as in Marketo, if leads are deleted, they are completely gone, meaning they completely get out of the radar, as if they have never existed.
You could also setup a sync filter so that, once deleted, they do not come back. Look here:
"Sync with Marketo" mysteries part 1
"Sync with Marketo" mysteries part 2
Syncing back of such leads is a potential way of creating duplicates. You could use the flow step of Delete Lead from SFDC itself when deleting these leads.