Transformer winding question

I made a joule thief the other day, slow morning =)
**broken link removed**

Everything worked out fine but there's a warning not to twist the wire too much as you're winding the coil. Now mentally this makes sense to me as twisted wire would be a coil wrapped around a coil. But I want to hear from the analog geeks here how exactly that would effect the coil and the operation of the simple oscillator.

I think the problem is mainly winding efficiency. If you twist the wires, they don't lay as flat, thus the wire takes up more room and have more air space between them. This can reduce transformer efficiency and increase leakage inductance.

Thanks you, seems to about sum it up.