Ice used in cocktails should be as clear as possible; however, the majority of ice made at home from tap water contains trapped air and minerals, which causes it to cloud up. Whilst this may be perfectly acceptable for opaque drinks, it can spoil the presentation of clear or colourless drinks such as a Martini.
If you make clear cocktails on a regular basis, it may be worth making a batch of ice specifically for this purpose using purified (distilled) drinking water, which is free of any contaminants that may cause the ice to become clouded. Although distilled water costs about twice as much as regular bottled water, it is a fairly minor expense considering the small quantities you are likely to use.