Tricky one this and the subject of some animated discussions between detectorists and archaeologists. For the detectorist the advantages are that you can search these sites almost all year round and finds will probably be in better condition due to lack of ploughing etc.
From the archaeological viewpoint – they would prefer detectorists to avoid undisturbed pasture or other grassland where there are likely to be significant archaeological deposits.
for example: if a field has been ploughed for twelve years, and then set to pasture, it would be seen as a responsible practice to detect on it since the artefacts are unstratified.