Your grandmother's belief may stem from older dishwasher models or personal experiences that have influenced her perception. In the past, dishwashers may not have been as efficient or powerful as modern ones, leading to the need for pre-rinsing dishes. However, advancements in dishwasher technology have made them more effective at removing food particles.
Modern dishwashers are designed to handle dishes with small amounts of food residue. In fact, many dishwashers today have built-in food disposal systems, such as filters or grinders, which help prevent food particles from clogging the dishwasher's mechanisms.
While it's generally recommended to scrape off large food debris before placing dishes in the dishwasher, there is usually no need for extensive pre-rinsing or washing. In fact, pre-rinsing can be wasteful of water and energy.
However, it's worth noting that some dishwashers may perform better with certain types of food residues. Items like egg, cheese, or starchy foods can be more challenging to remove, so it may be beneficial to lightly rinse those items before loading them.
Overall, following the manufacturer's instructions and guidelines for your specific dishwasher model is the best approach. Most modern dishwashers are capable of handling dishes with small amounts of food residue, making pre-rinsing unnecessary in many cases.