Corpus annotation “can be defined as the practice of adding interpretative, linguistic information to an electronic corpus of spoken and/or written language data. 'Annotation' can also refer to the end-product of this process” (Leech, 1997).
Each annotation that can be done automatically must be done automatically!
Why? Because revising is faster and easier than annotating... if the automatic system is "good enough".
Before using any automatic annotation tool/software, it is important to consider its error rate (where applicable) and to estimate how those errors will affect the purpose for the annotated corpora.