Flow Credentials: Best Practices?

I think that's the best bet, just to be on the safe side...... for now.  In a production environment, you don't want to be messing with accounts, etc.  That said, I'm very impressed with Flow and how it's worked so far.  With a little elbow grease and some time to figure it out, it's really useful and works well (and it's fast).  There's a lot of flexibility built in that's not obvious right away.  

That said, I'd love to see more documentation and examples.  I know I can do quite a lot but I don't know HOW without extensive searching around sometimes.  That's life with new services though, especially something as robust as Flow.

How Flow uses accounts is way more confusing than it should be. There is a lot of room for improvement and clarification in the documentation about exactly what account is going to be used, what perms are needed and how it going to show up. Sending emails is one area that needs a lot of attention.

We use several flow accounts for everything. It's somewhat easier to manage but also takes the guess work out of who an email is going to come from or who has access to what.  It's also handy for troubleshooting because we know if those accounts pop up, it's specifically flow.

Your flow account doesn't need to be global admin - it just needs access to whatever you are using it for. So if it is updating a list item, it needs that permission to the list/site. That said, often the flow account ends up with fairly beefed up perms. That's also another reason to think about centralizing the accounts you're using. I realize the context of the flow will run as whoever you add the connector as, but again, there's something to be said about knowing exactly which account to look for. I realize people will argue that a service account flow isn't as good for security but that really depends on how you implement it. I'd argue the other way, you know exactly which account does what and you be as granular as you need, knowing that account should always have access to specific areas and anything else is an anomaly to look at.

Don't worry specifically about E3 vs. E5. In fact, go cheap there and out the extra money into Flow plan 1 or 2 (premium plans). You will find that the premium plans give you not only more flows but a few additional kinds of flows.