You do not have to change anything when it rains! There's no need to change your watering cycle as your seedlings will naturally dry out after it rains. The silicone gaskets included with your Farmstand components will keep your electrical connections dry.
Check your electrical connections after a heavy rain to ensure that your seedlings will continue getting the water they need. Then be sure to check your water's pH level as heavy or sustained rain can affect your pH level.
If you have young seedlings and the weather forecast calls for more than 9+ consecutive days of rain with no sun, you may want to move or cover your Farmstand.