Most retail software assumes two decimals because most currencies have them. In Kuwait, Bahrain and Oman the smallest unit is a thousandth, and a system that silently rounds to two decimals will produce totals that do not reconcile. In Chile, Paraguay or Indonesia the opposite is true — showing ".00" on every line is noise and invites keying errors.
Change this before you start trading, not after. The setting governs how amounts are stored and displayed; adjusting it once you have months of transactions is the kind of migration nobody enjoys.
Separate question, often asked together: multi-currency. Supporting settlement in over twenty currencies is not the same as automatically converting between them on one receipt. If you take two currencies over the same counter, ask us specifically how you want that recorded before you assume it works the way you picture it.