Bullnose vs square edge stair treads
The building code does address the width, profile, and the rounding of the treads.

From the 2009 IBC:


  R311.5.3.3 Profile. The radius of curvature at the leading edge of the tread shall be no greater than 9/16 inch (14 mm).


I believe the reason for rounding the leading edge of the tread with wood is to reduce the likelihood of splintering. Also, standard soft pine building lumber is more likely to warp or be damaged with use than a hardwood stair tread.

Good luck with your project.