The only reason that you would need to do this is if your fill water has a high metal content. There are a lot of different metal sequesterants on the market and most of them are based on a phosphonic acid derivative. Get your water checked for metals (usually copper, iron, and manganese). If there are metals in your fill water then you probably do need to use a sequestering agent on a regular basis. Not that big of a deal really. They are probably having you use it right now since fresh plaster might be a little more prone to staining. I have a fiberglass pool and the builder had me use one when the pool was first put into service. Whether it was necessary or not I do not know. I fill with softened water from my whole house filtration system and there are no metals in my fill water. From people I have talked to about when their pools were built it seems to be a commen practice among pool builders to add a seqeustering agent when the pool is first started. Some of these products also bind up calcium so it might be part of the curing process for new plaster. I really don't know for sure.