Good advice Timbo. Telltales really are the ONLY way of being able to actually see how well (or badly) your sail is working. I have put 4 leech telltales on my main - I have 1 in the lower third, 1 halfway up and two near the top of the main about 2ft apart. (It's surprising, but sometimes one will fly cleanly while the upper one will twist all over (tip vortex?)) When the top two are flying clean the boat just FEELS fast.

I have 6 other tell tales around the sail and as Timbo says, you can't get them all flying, but even when they aren't they're still giving you information about the flow over your sail and that means you can make INFORMED adjustments.

While I'm still learning this new boat I'll use the telltales a lot. Eventually (as with previous boats) I'll know the boat/sail well enough to only refer to them to briefly check settings and be able to concentrate on speed and tactics.

In other words they are a tool to get a job done. Don't become obsessed with them or you won't be sailing efficiently.