Honestly, it all depends on the shader... although, I wouldn't recommend it. Shaders are grouped more loosely than liners. Therefore, you'll end up not getting a tight line.
I agree, There what they are and meant to do just that. I have used shading needles to back ground lines but they don't look as good and end up crappy over all.
Yes yes yes,and more yes.....anyone who says differently knows jack......I mainly use a 5 round shader for most line work as if you machine is properly tuned it lays a decent line(not really to worry if it isnt the best)because using the shader needle allows for more passes over the line for contours and thickness,and to smooth out shaky parts.....very very helpful....I also do a lot of lining with y mags as well,although harder but it is very useful as you can blend in the edges real nice