760+ on your middle credit score and you are true top tier credit regarding mortgages.
740+ is not the real 'bottom line' credit indicator if you are going to have PMI. The PMI rates are different depending on your score and loan to value. If you put 20% down, then 740 is the 'best' and anything above that is a perk.
I can give you the same fixed rate if you have a 650 middle as I can if you have a 750 middle credit score. The difference is going to be your loan level price adjustment which is based on loan to value and credit score.
Regarding open lines and maintaining your high score, there are a lot of different factors to consider.
As you mentioned: number of open lines, lines with balances, length of time, and type of line.
For the FICO, your score is made up of the following weighted items: 35% payment history, 30% available credit, 15% length of history, 10% type of credit,10% inquiries.
I would NEVER suggest closing your oldest credit card. I do suggest trying your best to keep your balances at 50% or less of your limit if at all possible to maintain high scores. I also suggest using at least one credit card per month to keep the line active. If you have an old credit card but never use it, it will eventually go dormant and not report even though it is open. Buy a tank of gas each month with your oldest line and pay it off before interest is accrued...you will see the positive results.
For anything other than a mortgage, which is the hardest and most in depth credit pull you can have, a 740+ score is going to be considered 'excellent' by anyone completing the financing.
Hope you find this helpful!
-Marc