Optimal = "just popping in and out" according to Bethwaite, but he assumes the reader knows what this means. I think he means supercritical: streaming aft, but with the sail trimmed in just enough that they occasionally pop behind the sail... much like Smyth's supercritical luff telltales.

Having just finished reading Bethwaite's book, I'm convinced I want to add two or three "leech ribbons" as the aussies call them. He points out that leach ribbons are a useful backup for when the luff telltales are not backlighted and cannot be seen. He does not advocate using them exclusively.

Bethwaite does have a diagram of a sail covered with telltales, but that's just to demonstrate which ones really matter: only the luff telltales.

He also mentions the fact that a properly trimmed sail will have a little turbulance due to air leakage around the leach. This is why they should be acting up a little when the sail is properly trimmed.

Bethwaite seems to advocate telltales or leech ribbons at different times, depending on the point of sail and sea state and wind strength. I'm going to have to reread to figure out when and why. However, using the two in combination seems like a great idea in most conditions because the luff telltales tell you about the size of the separation bubble (front-edge supercriticality) and the leech ribbons tell you about rear edge separation, and you want to be on the edge of both types of separation for maximum power, which requires getting the right camber (outhaul+downhaul+prebend) and the right angle of attack (sheet and traveller + helm). So, the leech ribbons do give some useful additional information not provided by luff telltales.

Also note that a small amount trailing edge separation is normal, but oversheeting increases the size of this separation, increasing drag and reducing power. The leech telltales indicate when this is happening, and large separation should always be avoided (except in light air for the bottom 2/3 of the sail...). In a breeze, you always want the leech ribbons streaming or "just popping in or out".

At least, that's my understanding after reading the book.