'Cheating?'
When all's said and done the making of a picture, whether the medium be oils, charcoal, needlepoint, wood-burning or photography, is a 'game' the picture-maker plays with him/herself.
When you set out to make a picture, you have a set of 'rules' in mind. These rules may be as simple as, 'I will use watercolors' or 'I will use 120 b&w film.' The rules may be more complex: 'I will use a digital camera, photograph a landscape which includes a single tree, shift the color balance so as to add a surreal note and use the sharpening function of the processor to the maximum degree possible.'
All of the above produce pictures which can be [and imho should be] appreciated on their own merits qua picture without regard to the technique used. When you view a classic oil painting, it doesn't matter a hoot if you know the blue was created by using ground lapis lazuli in an albumen medium. What you are impacted by is what the picture 'says' to you. You evaluate it on that basis. While it's fun to know the minutiae [He used a special lens with an f64 stop,] the impact's the thing.
So, if any means to create a picture is 'legal', what about 'cheating?'
Cheating, I should think, occurs if at all when you are not true to your inner 'rules'. Let's say you set out to take some street shots and your inner 'rules' include exposing the film and making the final print without using any technique beyond cropping the image. Then, when enlarging, you interpose a patterned screen, burn in the corners and post-process with a sepia toner. You have 'cheated' on your initial rules. That's all. Nothing more than that.
And to top things off, I really don't see anything wrong with changing your inner rules along the way if doing so will make a 'better' picture. [I'll leave the definition of 'better' for another lecture.]