It really depends on what access you want to give. You may have a situation where you only want to trust one domain and not all domains in a forest, or you may have many domains in a forest and a forest trust is the most convenient to setup and administer.
TechNet has a lot of good articles explaining the different options.
Most of the trusts I've setup have been for migrations and within the same company, but if it was to an external organisation I would give the least amount of access.