Not quite true, perhaps in 1.7, but it definitely shouldn't not in 1.8+. Provided the world type is Vanilla's, you'll see no changes aside from perhaps some extra flowers in newly generated chunks. However, there shouldn't be any world borders or anything like that.
Although, i'm not sure why you'd use the mod if intending on using an existing world, unless of course an NBT editor was used to change the world type (causing seams). That might be what you're referring to though in which case my apologies.