There is no “pure” water. Naturally occurring water is not pure, whether it comes from underground wells, surface reservoirs, or even freshly fallen rain. This is because water is a very aggressive solvent and picks up minerals from the ground or gases from the air.
For cleaning of gross amounts of contaminants at an early stage of assembly, tap water may be adequate, provided that the seasonal consistency is acceptable. Keep in mind that if the process is moved to another location, the tap water may be different. Also, because dried-on contaminants tend to be more adherent, any impurities in the tap water that remain when the part is dry may be more difficult to remove at a later stage of operation. For general cleaning, perhaps a single stage DI or RO system will provide the purity and consistency needed.
Many high performance applications employ systems that incorporate more than one treatment process, or have a multi-pass system to increase the purity. Any system must be properly monitored and maintained.