Cleansing crystals is always a good idea. Crystals have the ability to hold, or to 'absorb' and redirect different energies. You should make sure to properly cleanse your crystals after buying them as they can hold negative energies from having been handled by people. They also may have traveled far before arriving in the shop where you have bought them, so it is strongly recommended to cleanse your crystals to ensure that any potential negativity, or energy misaligned with your own, is removed.
Water has its own healing property, and can neutralise any potential negative energy absorbed by your crystal, returning it to Mother Earth. It is said that natural running water is the most effective way to cleanse your crystal, but you can submerge them in any clean water in order to neutralise negative energy. However, tap water, which is likely to contain chemicals, is not advisable. Mineral, filtered or sea water would be best used where possible.