Sequestrant binds to metals in the water and prevents them from staining the plaster. It also helps keep calcium scale from forming, calcium is really a "metal" in this context. Fresh plaster has a tendency to raise the PH very rapidly, which tends to cause calcium scaling, which is very expensive to remove. If you are really good about keeping the PH under control, there is no need for sequestrant, but if you miss a day, having sequestrant in the water can save you some expensive and unsightly calcium scaling. (Don't take this as an excuse to ignore the PH, the PH still needs to be kept under control even with sequestrant.) As long as you have copper or iron in your water you need to use sequestrant regularly. If you are using it to help control calcium scaling with fresh plaster, you only need to use it for the first month or six weeks.
An acid start is simpler than a conventional start, but it can cause some minor issues if you have a heater with a copper heat exchange coil that you can't bypass while the PH is really low. Normally it is best to leave this up to the plaster people. Frequently the plaster people will specify their preferred startup technique and require following it in order to honor the warranty.