You could superglue them to the top of the case if you really wanted to, it doesnt matter, cable ties or zip ties are more common cos they are sometimes coloured to suit the theme of the PC, but anything can be used.
*EDIT* - also, remeber that plastic or any type of rubber is an insulator, it wont conduct electricity even if you shoved it in a power station. So you can put cable tiwes and electrical tape anywhere in your system.