While HBase provides a lot of in-built functionality to manage region splits, it is not sufficient to ensure optimal performance. The number of regions in a table, and how those regions are split are crucial factors in understanding, and tuning your HBase cluster load. You should monitor the load distribution across the regions at all times, and if the load distribution changes over time, use manual splitting, or set more aggressive region split sizes.
Good article from HortonWorks to help you get started on Region splitting.
http://hortonworks.com/blog/apache-hbase-region-splitting-and-merging/#.UkK0E4YW12M
