Nice question VF. I love how everyone can have such different opinion and techniques, what great machines.
I like the basic three points mentioned above:
1. Best performance per power setting is most likely in trim.
2. If using the ball, check in hover and use that as your "basic reference" but understand that things like cross winds (hover), fuel burn, and speed all effect and can change your CG (which way is down).
3. When in doubt use the strings, if they don't exist nothing like some scotch tape and yarn.
I believe the first pilots to fly in clouds many years ago (before "modern" instruments) achieved this by taping a single strand of string, with small weight, on the ceiling of the cockpit.
So the string and ball (weight) is one of the first attitude references used in aviation.