I personally think that Grass should be offensively neutral against Bug, but not super-effective against it, since Bug is already super-effective against Grass because insects eat plants.
I think Grass should be neutral against Bug because some plants like Venus flytraps and pitcher plants eat bugs, hence Weepinbell and Victreebel are each known as the Flycatcher Pokémon, and Carnivine is known as the Bug Catcher Pokémon.