At the moment my hair is so long that I need to either spend an hour straightening it layer by layer, or if I try letting it dry naturally (curly) it just turns to frizz as my hair's too heavy. Oh and 'letting it dry naturally' involves using lots of products and regularly scrunching it over about 2 hours until it dries - so not that easy.
So shorter means it will naturally take less time to straighten, and should mean that the lighter layers make my hair curl much better. Although I'll still make sure it's long enough for me to stick up in a ponytail, although may involve lots of clips!!
I've never done a strand test before, just winged it every time - I've been really lucky. I remember as a kid, about 13 all the girls started using 'sun-in' spray dye and their hair was going all sorts of colours, green, orange etc etc.
I used it and it worked really well. So I guess I have just been lucky!