Of all the components on your barbecue that might need to be replaced someday, the grill grate is one that will need replacing at least once over the life of your grill. Your cast-iron grates may have corroded over winter, for example. Perhaps your porcelain-coated grates are chipped. Damaged grates can contaminate food, affecting not only its taste but also the health for those eating off the grill.
A small amount of rust on a grill grate won't cause you too much concern. Brushing with a wire brush should remove any surface rust and provide you with a clean surface. Only worry if the rust penetrates deeper into the grate and won't scrub off, or if flakes of rust stick to your food. If that happens, replace the grate immediately as rust not only makes your food taste bad, but it should not be ingested for health reasons.
Chips on the grates create an uneven cooking grilling surface. Grates covered in porcelain are particularly prone to chipping if you bang them or even scrub them too vigorously. A chip on a coated grill provides an entry point for moisture, increasing the likelihood of rust.
Sometimes grates get so dirty that you just can't clean them anymore. If you have soaked, scrubbed and brushed your grate, but you can't get it clean, it's time for a replacement. Debris accumulates because the grate hasn't been properly cared for. Grill grates require regular cleaning and brushing to remove debris and stuck-on food.
You don't always have to replace a grill grate because it's damaged. Many people change their grill grate because they're looking for something else out of their grill, but love the barbecue itself. For instance, if you want better grill marks on your meat or food to retain heat for a long time, switch to a cast-iron grill. If you want a low-maintenance grate, switch to stainless steel.