Mark Rosen of the University of Michigan talks about how he likes his team to have a variety of serving techniques, speeds, locations and trajectories. This will keep the other team on their toes, forcing them to adjust to every new server that steps on the line.
If you are coaching an older team where players have already developed their serves, tweak their serving technique to improve it, but allow them to continue using the type of serve they are most comfortable executing. Multiple serves can be powerful tools: a clean float, a jump float, a topspin, and many other types. Allow your players to hone their personal serve.