Stock sbc are 23 degrees meaning the valves are inclined upward from the axis of the cyl bore 23 degrees. So yes a 12 degree head has them angled 12 degrees. Hypothetically if the valves were 0 degree they would be parallel to the bore.
Don't confuse this with brodix's -12 head. Idk why or how they came up with that but they've had -8,-10.11 & 12 among others. I believe the -12 was among their first to deviate from 23 degree and I think they were 15