An important aspect to remember is that restorations e.g. Crowns, fillings, veneers, etc. cannot be whitened, so it is very important that this be addressed before whitening commence. If the desired color of the teeth has been established by the whitening process, the restorations can then be altered to match the new white color of the natural teeth.

So, to answer the question, there is definitely a wrong and right, with the right system in the right-hand great results can be achieved but the wrong system and incapable hands will have dramatic effects including post-op sensitivity, burning lesions on gums, and even sunburn from the extremely sharp UV light.