When teaching reading, it is best to use a systematic approach that includes: phonemic awareness, phonics, vocabulary, comprehension, and fluency in your instruction.
With regards to phonics in particular:
“Systematic and explicit:
- phonics instruction is more effective than non-systematic or no phonics instruction.
- phonics instruction significantly improves children’s reading comprehension.
- phonics instruction is effective for children from various social and economic levels.
- phonics instruction is particularly beneficial for children who are having difficulty learning to read and who are at risk for developing future reading problems. (italics mine)
- phonics instruction is most effective when introduced early.”
(Taken from: http://www.ed.gov/programs/readingfirst/guidance.doc )