I can't talk about Marketo usage specifically as I am new to the product, but I am not new to digital marketing and connections to SFDC with marketing automation platforms. So, if you like, here's some learnigs you may find useful:
1. We started our marketing automation journey putting only MQLs in SDFC and not too long dow the road found this was limited and created data problems. People have so many ways these days to get into systems, waiting until they scored as an MQL created more duplicate entires and data problems.Also limited view of sales people/telequal on all touches when someone chose an altenate contact method.
2. We utimatey set up ways in SFDC to trigger immeidate MQLs...someone fillled out a specific web form, sent a direct email asking for contact etc, or called an 800 number promoted in a marketing automaion process. This allowed us better attribution of all types of contacts and gives the customer control over contact method. More meaningful reporting was a huge side benefit.
So long story, all "known" contacts was what we chose as best practice to flow into CRM. There are costs involved with this decision and other pros/cons, but this worked best for us and really better connected sales and marketing.Of course, you'll have to look at your processes and tools and make the right decision for you.