Usage of pipeline bridge and clock crossing bridge

Here's the response I got from the Altera support regarding this questioin:

There is no a rule for how to place the pipeline bridge and the clock crossing bridge, this is all depend on your design requirement and how are you going to optimizing the system. You can refer to the following link page 8 (“using Bridges”).

http://www.altera.com/literature/hb/qts/qsys_optimize.pdf