Material based on pitch

No one seems to agree with me when it comes to what material i use based on pitch.

A. What is the range of pitches that you put your flat systems on?

B. What is the range of pitches that you put 3-tabs on?

C. What is the range of pitches that you put architects on?

Is there any wiggle room based upon underlayments?

Is there any wiggle room based upon what the homeowner wants?

0/12-2/12 flat.
2/12-4/12 3tab
4/12-12/12 dem.
no.
no.

gweedo.