In my opinion, I dont think it matters much. However, i do like my urls to have keyword placement in the same manner as they would be typed by users (so, basically keywords with more search volume or what you think people would be typing in)
To answer your question, I would ask what do you think people are going to type in Google for if they wanted any of these services? Will it be 'whitening treatment' or 'treatment whitening'?
As Logan said, its not going to make or break your SEO, I wouldnt be too worried about it but yes, when being in a situation like this, I would like to go with what I mentioned earlier.