Grain boundaries in epitaxial graphene on the SiC(000 (1) over bar) substrate are studied using scanning tunneling microscopy and spectroscopy. All investigated small-angle grain boundaries show pronounced out-of-plane buckling induced by the strain fields of constituent dislocations. The ensemble of observations determines the critical misorientation angle of buckling transition theta(c) = 19 +/- 2 degrees. Periodic structures are found among the flat large-angle grain boundaries. In particular, the observed theta = 33 +/- 2 degrees highly ordered grain boundary is assigned to the previously proposed lowest formation energy structural motif composed of a continuous chain of edge-sharing alternating pentagons and heptagons. This periodic grain boundary defect is predicted to exhibit strong valley filtering of charge carriers thus promising the practical realization of all-electric valleytronic devices.