Leaving rice in a rice cooker on the "warm" setting for an extended period of time, such as all day, is not recommended from a food safety standpoint. The warm setting on rice cookers is generally designed to keep cooked rice at a safe temperature for a short period, usually a few hours.
Leaving rice at room temperature for an extended period can create an environment conducive to bacterial growth, particularly the growth of Bacillus cereus, which can produce toxins that cause foodborne illness. This is especially important in humid climates where bacteria can multiply more rapidly.
To ensure food safety, it is generally advised to refrigerate cooked rice within two hours of cooking. If you want to keep rice warm for a longer period, it's better to transfer it to a shallow container, cover it, and store it in the refrigerator. When you're ready to consume it, you can reheat it thoroughly.
It's worth noting that cultural practices can vary, and some people may have different preferences or methods of handling rice. However, from a food safety perspective, it is generally recommended to follow guidelines to minimize the risk of foodborne illness.