Psst, ask her what she enjoys most! 
Breast sensitivity varies so much from woman to woman...how sensitive they are is going to determine how much pressure (from sucking, squeezing, nibbling) is best for her. My boobies fluctuate in sensitivity, from very to off-the-charts. There are times when a light caress is all I can withstand! And there are other times when I want groped and my nipples pinched. So, you see, there are no hard-and-fast rules here.