The question is how you acquire the new words. If you learn words from reading, you’ll have a better sense of their nuance than if you learn them from a thesaurus. Thesaurus writing is often easily identifiable thanks to the writer not catching the full meaning of the word. Some authors are better than others for challenging one’s vocabulary, notably Anthony Burgess and Cormac McCarthy. While I enjoy David Foster Wallace, I feel like he had a tendency to abuse medical terminology in an attempt to challenge the reader with unfamiliar vocabulary.

If I were to contemplate using a word I found in a thesaurus in my writing (and it was something that I didn’t already know), I would make sure to (a) read the definition in a dictionary and probably do a search on Google books to see the word in action.