A certain amout of carbon build up is required for your car to run well but it's when your car gets choked up with it kills the cars performance and fuel economy.
Once a year will do no harm but after that you'll only do damage to the life of the engine.
As for bug spray and tar remover, it is dangerous. It states on the back of the cans to only apply lightly and leave for a few mins out of direct sunlight. You will damage your paint work if used alot and it affects different colours, metalic makes a difference. Even different car makes have different reactions. Some manfactures use more layers of paint or more lacker etc. So it's just worth keeping that in mind aswell.