Email Bounces

I usually don't investigate or anything. If it's a really important email or an event we are pushing to a list of specific ppl, I'll pull those that were blocked and send that list out to sales. Then they can reach out to them individually.

That's usually a good practice. It depends on the number of people and how engaged they are. Do you really want to ask Sales to call up people who are likely no longer there or perhaps don't want to be called?
 

Another method I recommend is to go through the Hard Bounced list using a smart list. See if you can spot obvious mispelligns of domains or names. A lead will often type in their email quickly and not realize they transposed letters. Fix that and reset the Email Invalid flag. then you can resend to that person or group.

For our paid subscription email broadcasts, yes I track those every week or monthly because that's how often we send out those emails.  If it's a case of our emails being blocked because of spam, I reach out to Marketo support if it's our emails or another company from the shared IP that caused the problem.  I do give the invalid emails to our sales people because they have to get a replacement email.  Even those who are doing a trial subscription, they have to call the people whose email address is invalid.  Ditto for the blocking emails for the trial subscribers because that affects their ability to convert them to customers. 

For bigger promotions, any bounces that are invalid, I send to our list developer to investigate.  Any blocks that are promotional, unless they are a paid subscriber, I try to give it to our sales or our list developer to investigate.