This article describes how Collaboration Services picks the global catalog (GC) server to connect to for active directory (AD) information.
Collaboration Services connects to the nearest GC in the domain, where the computer on which Collaboration Services instance is installed, resides. It does not really "choose" if there are multiple GCs - rather it is Win LDAP which chooses the GC to connect to.
No particular measures should be taken in a correctly functioning environment, as Exchange 200x itself heavily relies on using GC. Below is an explanation why.
Exchange 200x refers MAPI clients to the nearest GC server. Clients use the GC to access the GAL, and basic Exchange 2000 functions such as message routing won't work without a GC. In Windows 2000, the GC holds user, contact, and group details for the entire forest, so in an Exchange 2000 deployment, you need to place GC servers in network proximity to your Exchange servers.