The same rule for watering in containers holds true for watering in a raised bed garden. You water when the bed needs it. And you test that by using your finger - touch the soil, if you finger comes away damp, the garden is fine. If it comes away dry, then you have to water.
The confusion normally happens because a raised bed will normally use more water than a standard ground-level bed assuming all things are equal. In other words, if the soil is the same, a raised bed will use slightly more water than a ground level bed. And soilless mixes will generally require more water than ground-level soil as well so if you're modifying your soils, you have to pay attention to the moisture levels.
Why? More evaporation from the sides of the beds primarily but there's also changes in the downward movement of water due to soil "wicking" in a container raised above the ground (particularly if the soil has been modified a great deal) It's in the physics of water movement. But in my opinion, this isn't important stuff to know - just understand a raised bed will use more water and be done with it. Then you can sort out how much more your soil will use.
So - you do have to water a raised bed more than you do a ground-level bed. That's the bottom line here.
And when you do of course, the water is going to dissolve the nitrogen and take it with it on the way down the bed. So you have to replace this nitrogen on a regular basis, as in every week, to get a great yield of vegetables or flowers. This is particularly true when you're using a soilless mix instead of garden soil. Soilless mixes do not have the same mineral and micronutrient content contained in regular garden soil so you have to provide it.
So that's the easy method of figuring out the watering in a raised bed - it's all about giving the bed the finger. ;-)