So what is the current consensus on how trunks should be setup ? In cisco catalyst best practices doc for catalyst 4500/6500 it reads the following.
Cisco recommends an explicit trunk mode configuration of dynamic desirable at both ends. This mode is the default mode. In this mode, network operators can trust syslog and command-line status messages that a port is up and trunking. This mode is different from on mode, which can make a port appear up even though the neighbor is misconfigured. In addition, desirable mode trunks provide stability in situations in which one side of the link cannot become a trunk or drops the trunk state.
Other people are saying you should force them on . Personally I don't like forcing them on because you never really know if the trunk is working corectly. All it takes is a layer 1 link and the port will say it is trunking when it very well may not be working or possibly blackholing traffic if not configured properly. At least when you use dtp you know the trunk is correct.
Best Practices aren't bibles. As long as you understand the pros/cons on a design before implementation, it's what really matters.
There isn't a right/wrong way of using 'on' vs 'desirable'. I just wanted to pointed out a document that contained information regarding the use of the 'on' option and why.
If you are using 'desirable' and you are happy with it, then you should continue doing so.