Welcome to TRF Bert:
First there really is not such thing as "best".
As with so many things about our hobby It really depends on what YOUR intended use is for the Streamer, and in what type model it's being flown.
Are we talking about Sport flying or Compeitition type models?
What Materal....Mylar, Crepe paper, Wrapping paper mylar, Space blanket, Tracing paper,Teflon tape, Surveyers tape or Rip-stop nylon?
For most cases attachment is at the end of the strip, but not always. If we're talking about compeititon type accordian folded mylar streamer that end attachment may be in the center of the leading edge or along one side.
Some Sport flight surveyers tape steamers are simply tied in the middle directly to the shockline.
Here are a few examples of streamers I've made...all are end attached, some center mount, some corner mount but that just the way I make them...others do lots of other things
The 10:1 length to width suggestion is just that; a suggestion. it's really a rule of thumb form back in the day when crepe paper streamers were the main material in competitions. Depending on the material much longer lengths can & do work will, My Estes Goblin D powered bird flies with a 2" x 120" crepe streamer all the time.
Hope this helps a little.