There's a lot of ISO standards (and IEC and ...). When you say "need ISO" - do you mean ISO 13485? FDA has adopted by reference.Is your device a (regulated) medical device (wellness devices get into gray areas and there's often a tendency to claim you're not a medical device just by asserting you have a wellness device.)
You're never legally bound to comply with any standards, even if you have a medical device that must meet regulatory requirements. Complying with relevant standards is the most logical path, in most all cases. Regardless, if you're putting a powered device on the market, you will likely want to comply with any relevant (e.g., electrical) to provide some assurance of basic safety.