There are two reasons to thin clusters. The first is because the vines are overcropped and there is insufficient leaf area to ripen the crop to the desired level. This condition is usually recognized by poor shoot growth. In this case, removing fruit as early as possible will give the vine more time to ripen the remaining crop. A second reason for removing crop occurs when the crop and leaf area are well balanced but grower or winemaker hopes that wine quality can be improved with less crop per vine. In this case, the thinning is usually done at veraison (color change). When most of the crop has turned color the remaining green clusters, usually 20 to 30%, are removed. The logic is that removing the later ripening clusters will increase average ripeness of the remaining fruit through harvest and improve uniformity of fruit composition. While this type of thinning is common, it should be noted that there is little research data to support the practice.
As to when during the growing season, thinning should be done at or around bunch closure (when berries are first fully touching within the cluster). Earlier thinning usually leads to larger berry size on the remaining clusters and is likely to negate at least a portion of your efforts in crop reduction. Other than green thinning at veraison, later thinning may not be effective in altering fruit composition and ripening rate.