When I started airbrushing someone had given me a couple of boxes of 10A's so that's what I used, it wasn't until I started college about 5 years later I realised there were other ones available ( well before the internet - it's so easy now ! ).
My tutor who was an airbrush god said he used 11 so i tried them, he told me about seeing the tip of it and staying on your mask lines. We used to ink onto masking film with a .18 or .13 technical pen, he said it was better as you had less room for error than if you followed a pencil line which however sharp your pencil was still gave you room to wobble. To be hones that's something I struggle with now. Working to the lines of even a fine marker pen aren't as accurate as I want them to be because your scalpel blade can wander within the width of the line, am I being too picky, I probably think I am....
Aside airbrushing at college we used to ink on film a lot, if we made a mistake we were told the curved blade was best to scratch out with as the pointy ones made furrows in the film.
Tel - when you say a straight edge blade do you mean something deadly like a razor blade or a stanley blade or do you just mean straight edged scalpel blade ?