Safety stock is definitely

Safety stock is definitely considered for inventory level planning and it was a popular concept before MRP. It’s more useful in reorder point planning and was used extensively in 70s/80s.

JIT /Lean manufacturing try to make safety stock zero and that does help the margin of the plant.

MRP mostly considers on hand and scheduled receipt. Though you can use safety stock but then it undermines the concept of using a good forecast & planning. Now a days no big MNCs allows to have big safety stocks (say more than a week).

Again it all depends on the manufacturing plant, the business strategy and demand level – stable /unstable. But at the end the whole idea behind using a good ERP is to cut the cost, have less safety stock and reduce the lead time.