Many cookbooks say "never," but that advice can run counter to sound hygiene and pleasing taste. Unless you use these metal utensils daily, they should be washed briefly with a little soapy water (then rinsed and thoroughly dried) in order to rid them of excess surface oil. Otherwise, the surplus oil will become rancid within two days or so, giving cooked foods an off-flavor. Washing your wok or omelet pan gently with a cleansing agent need not ruin your prized possession.
Some cookbooks recommend an alternative method: Scour the pan with dry salt. Since that technique can also chemically precipitate rusting, it is not advisable to follow that technique.