The only time I use a thesaurus, or the synonyms option, is when I am thinking of a certain word but cannot quite find it. The thesaurus can help to jog my brain so I can find the word I was looking for. I think it is best to use the words that first come to mind because they are often those best fit for the job. I have read many papers where people have obviously looked up synonyms and didn't know what the word meant but used it anyway. This can make someone look very unprofessional, so I don't recommend using any words that you have not heard or seen in context or that you are not sure you have a firm grasp of.