|Are you talking about what is now called "standard" editing? If it is obviously a texture, then yes. Such as my recent freestudy entry, where I added textures using Nik analog Efex. I felt I was pushing the use of the texture in this shot, but it performed mediocre and nobody challenged it. Maybe that was responsible for some of the low votes.
You can't add photographic elements to the shot, however, such as if you wanted to place a person on top of a mountain that wasn't in the original shot. That is allowed under extended editing, which pretty much lets you to do any photoshopping you like, as long as the source images are within the allowed date range.