Citing instances where ADS-B Out has been inadvertently left off or activated in the wrong mode, FAA has suggested in a recent article that pilots add a step to their preflight checklist regarding ADS-B. The item they recommend adding is “check that your ADS-B system is on,” and they recommend it for all flights, regardless of the airspace in which you intend to operate. Their recommendation is universal because while ADS-B Out is not required equipment in all airspace for all aircraft, when ADS-B Out equipment is installed, it generally must be operated at all times during flight even where not otherwise required, per FAR 91.225(f).
If the output from your ADS-B system is absent or nonconforming, you are likely to receive a letter from FAA’s ADS-B Focus Team, something I’ve written about before. This is true even for ADS-B Out systems installed in aircraft that don’t require them, such as aircraft originally certificated without an electrical system per FAR 91.225(e). Battery-powered ADS-B Out installations are required to meet the same performance standards as other ADS-B Out solutions (see FAR 91.227), and if the output of your equipment is out of spec, FAA is not going to ignore the nonconforming performance just because you were not required to equip in the first place.
Another reason FAA recommends confirming your ADS-B Out is active and transmitting all required information is to ensure you are visible to other aircraft and get the most utility out of ADS-B. ADS-B ground station transmissions depend on aircraft with ADS-B Out to activate them, so aircraft flying with misconfigured (or inactive) ADS-B Out might not see the whole picture. Additional information to familiarize yourself with ADS-B installations can be found here.