In general tiles are placed after existing tiles in the default layout. Within that general rule there are several additional factors when evaluating initial placement.
More mission critical tiles - such as GauchoSpace or GOLD - will be considered "first tier" tiles and need to be near the top. Tiers are broken down as such:
Mission critical or high value tiles. The first nine tile spaces are reserved for this tier. Any tile that spills out of this will either offset the start of Tier 2 or lead to an assessment of existing tiles.
Important but perhaps not core information. This is the next nine tile spaces.
Any content that doesn't easily fit into the above tiers will be placed in this tier.
If a group has more than one content tile in Bridge they may request they be placed adjacent to each other. Such requests will be honored as best as can be.
Tiles that have little or no usage, receive significant negative feedback, or are generally abandoned by their group will be downgraded to make way for more popular tiles.