What's your typical decision process like for deciding whether to use No. 2, No. 1, Select Structural, etc. when specifying Southern Pine? Obviously less cost is preferable, but higher quality wood will need smaller members. Is it different for every job?
use SPF for most framing, especially walls, floors and ceilings with drywall attached. Using SP for headers if SPF doesn't work is fine as the dimensional changes don't have a big effect on the lower members. In our area (central and northeast PA) DOug Fir has become a more economical choice vs SP. SP is commonly used in truss designs where the member will not have a brittle type finish applied.
Unless you plan on conducting a lot of site visits at some point what's most commercially available is what will be used. It's best to design with that in mind, in my opinion.