Alasdair, great question. It is true that everyone will tell you that constraints should be avoided, but they are there for a reason and should be used when it makes sense. The key here is that the schedule should follow your requirements without contradicting it. Lets take a look at typical training requirements. They are sometimes scheduled to occur at a best timing in the schedule in the beginning (with predecessors and successors), but once booked they are fixed. Timing within the schedule doesn’t matter, and is now independent and a priority over other activities for those attendees. The training class must start on a specific date with no predecessors or successors…it stands alone.
Now the one consideration is the people you have scheduled in the class and how that class affects their schedule. That will need to be managed and dependent on how you are managing the schedule as a whole (from manually scheduled to resource leveling). That is a different question. Hope that helps.