It is true that many women report that fenugreek can help improve breast milk production. Fennel is another herb widely thought to increase breast milk production. The reality of the situation is that other very important hormonal process in your body need to take place to ensure adequate milk supply. In addition the feeding behaviors of your baby are also a major contributing factor that will determine your milk supply.
Both of these herbs are recognized to be safe during breastfeeding so I think it is certainly worth a shot to see how your body will respond. If you have had extreme difficulty producing an adequate milk supply it is unlikely these herbs alone will fix the situation. Make sure you are taking good care of your body by eating adequate amounts of nutrient rich food to meet your increase caloric needs and staying well hydrated by drinking plenty of filtered water.