I never seem to have shrinkage problems with wood that reflects through
the sheetrock as a crack. I use off the rack 2X material, and build a
sandwich beam by laminating the 2xs on both sides of 1/2 inch plywood
and plenty of glue.

Most long term cracks I have seen (that weren't caused by movement)
when we pull out old walls are from 1) the header being sized to
minimum structural requirements instead of one size over, 2) the
headers didn't have the 2xs crowned properly, leading to the worst
offender, 3) headers were not assmebled properly so that they sat dead
flat in the framing causing them to move over time.

If you are worried about what size wood to use, your library or B&N
will have plenty of books with span tables in them detailing nominal
lumber size.

Use plenty of nails and glue, build your header correctly and you won't
have a problem with regular yellow pine. Unless specified differently,
that's all we use.

Robert