You should perform all of the steps of each upgrade between the version from which you are starting and the version to which you are upgrading. You do not need to install each intervening version, but you do need to carry out all of the configuration changes and additions, and all of the database updates, for each one. For example, when upgrading from 1.6.x to 1.8.x, you need to perform the configuration & database upgrade steps detailed in Upgrading From 1.6.x to 1.7.x followed by those detailed in Upgrading From 1.7.x to 1.8.x.