This depends on what you are doing. If adding a new domain new tree to an existing forest or child domain to the existing domain,tree,forest.A child domain the trusts are automatically set up.A new domain new tree same forest are supposed to be automatic as well.If the first DC is the DNS server and you join the forest I have found it a lot less hassle to create the forward lookup zone in advance for the new domain.Then when the domain joins and the four subdomains are present(_mscds,_sites,_tcp,_udp)you can make sure the trusts are present in domains and trusts.
You can add trusts in the same area but start with the trusted domain first then the trusting domain.You can by clicking on the trust and selecting edit get an option to verify the trust which will quite often find and fix problems with a trust.