Are stopwords helpful when using tf-idf features for document classification?
From the way the TfIdf score is set up, there shouldn't be any significant difference in removing the stopwords. The whole point of the Idf is exactly to remove words with no semantic value from the corpus. If you do add the stopwords, the Idf should get rid of it.

However, working without the stopwords in your documents will make the number of features smaller, which might have a slight computational advantage.