Be careful with that - unless you're talking about something like a solar
eclipse, just because a task *should* start on a certain date doesn't mean
that it absolutely and positively WILL or even CAN start on that date.
Using a Must Start On constraint means that the schedule will always show
that task happening on the specified date, period, even if there are other
factors that would make it impossible for it to really happen that way.
IMHO, one of the most valuable uses of Project is as a tool to predict
whether your project will be successful or not if you do the work the way
you've outlined it in the plan. For my money, if a task is going to pushed
past an important deadline because I haven't assigned enough resources to
tasks up the chain or some such reason, I want the schedule to tell me that
rather than just parroting back what I hope can happen. To do that, it has
to show the consequences of my decisions. Contraints such as MSO, MFO,
MFNLT, etc lock the schdule so those tasks *always* show happening on the
required dates and it won't alert me in advance when things are going wrong.
