There is evidence from Harrison and Bates (1993) (Circulation vol 87, p 1461) that GTN dilates larger coronary arteries better than smaller ones. This would have little effect on the overall resistance of, and flow through, through normal coronary arteries (since resistance mainly resides in the arterioles which are not dilated), but could increase flow through coronary collaterals, thereby improving flow in stenosed coronary arteries.
