That is a loaded question and depends on a lot of factors including what your intended use of credit is. What your income is. What your rates are if carrying a balance. What your cash reserves and savings are. If you are adding to reserves and savings.
Personally, I tend to carry more balances than many others. But I personally don't like a tweaked FICO at all times. I like a healthy FICO.
If you are seeking the best FICO: then it is straight forwards, 1) low reported balances/utilization (under 7%); 2) low number of accounts with a balance (preferably no more than 3 including home and auto); 3) a small reported balance on 1 card (like $10 to $50) for useage
This will optimize your FICO. However, if you maintain this profile, then you may find AA if it ever changes much. So if you go a long while always PIF, never a balance, always tweaked FICO, then you may get flagged if that changes much in the future.
Personally, I exercise my credit, not just for keeping it open, but to keep it flexible. But then again I have a business and often have uses for credit in the normal cycle of cash flow.
The best rule of thumb for debt: Don't have any. But of course you must have debt or a history of debt in order to get credit. Don't finance anything that should be paid by cash. Pay cash for those things you could finance whenever possible. Make sure the "return" is greater than the cost when financing.
Good luck!
Starting Score: TU 766 EQ 788