I did some research and found a lot of discrepancy among recommendations for the decimal precision when doing geocoding. Some say you only need 4 digits of precision and others suggest up to 15. In my own testing the best results seemed to be with 6-digits and SQL type of PRECISION(9,6) for latitude and longitude. This should provide a resolution of less than 12cm which should be sufficient for most purposes. The storage requirements should be 5 bytes in MySQL and 9 bytes in PostgreSQL.