A central design recommendation for both content types and term sets is the Open-Closed principle: open for extension, closed for modification. The Open-Closed principle states "software entities (content types, term sets) should be open for extension, but closed for modification"; that is, such an entity can allow its configuration to be modified through extension without altering the core base entity definition. This is especially valuable in a production environment, where changes to base definitions will necessitate code reviews, unit tests, and other such procedures to qualify those changes. The principle isolates the changes to the extension entity definitions, because the core base entities are immutable. This makes it simpler to evolve the entities in a controlled manner while keeping a common, consistent core base definition.
SharePoint is restrictive on what changes can be made when evolving or specializing content types, with very limited support for modifying existing content type definitions. The recommended practice is to use a base set of core content types (the closed part) and then perform all specialization and variations on content types inheriting from the base content types (the open part). Evolving term sets is less restrictive than content types and allows for much more flexibility in changes to their definition, including renaming and merging terms.
New in SharePoint 2010 is the capability to share content types and term sets across multiple site-collections and even across farms. These enterprise content types and term sets can be combined with local site content types and local term sets, the giving a combined syndication effect. The Open-Closed principle can be applied also to this syndicated taxonomy: use the enterprise content type inventory and the enterprise term set inventory as the core taxonomy base (the closed part), and use the site inventories to extend the taxonomy with local specializations (the open part).
In technical SharePoint 2010 service application terms: create a primary term store and a primary content type hub, then extend those locally with additional local term sets and local content type inventories subscribing to the primary core taxonomy.
Applying the Open-Closed principle both for content type and term set definitions, and for content type and term set syndication, allows for defining and enforcing governance policies that makes it possible to have centrally controlled lifecycle management of the core base definitions of your SharePoint taxonomy, while still allowing local extension and adaptation of the taxonomy.
See SharePoint 2010 Intra-Farm Term Store Syndication for details on realizing taxonomy syndication, including configuration of permissions per web-app, user or group for adding or modifying term sets.
