Depends on what you are using. Most granular fertilizers are sprinkled on or worked into the soil then watered. If you are using a water soluble fertilizer, you are watering at the same time you are fertilizing.

in my z5 .. we are 2 to 3 weeks.. before last possible frost ... i wouldnt be juicing my plants too early ...

as such.. if you insist.. no need to water it in ... it will get rain sooner or later ...

ken