John Fisher talks about the use of bitter apple in his book and says it's fine to use.

"I use a particularly obnoxious substance called 'Bitter Apple'. It can be used for stopping dogs chewing bandages or licking wounds but it is particularly effective for stopping them chewing anything, if the introduction is done in the right way.
First of all you need to obtain a cheap perfume which should be diluted quite considerably and then put into a spray bottle. Then you need a bottle of bitter apple and a tissue. Soak the tissue in bitter apple and put it on the floor. A few inches in front of this tissue, spray the floor with the diluted prefume and call your dog over. As it arrives at the tissue, it's highly developed olfactory system will have registered the scent of the perfume. Pop the tissue into its mouth and hold it shut for a few seconds before allowing your dog to spit it out - which it surely will. Your dog will then sneeze, salivate, spit, rub its nose on the carpet, do anything to get rid of the awful taste.
The bitter apple can then be sprayed upon any surface that you want to protect (except polished surfaces). A few inches from what you are trying to protect, your diluted perfume can be sprayed.. Your dog will approach, register the smell of the perfume and remember the taste to which it was a prelude. The perfume can then be used as a trigger warning to stay away from anything you do not want the dog to touch, even if it is not protected by the taste deterrent. Eventually, the smell of diluted perfume will disappear as far as we are concerned, but the dog will detect it for a long, long time."