This reduces breakage when you undo the cornrows. Do this on a regular basis. You could use rinse-off conditioner or leave-in. Dilute the rinse-off conditioner so that it is easier to use between the hair. When it is diluted, it is easier to rinse off.
When not diluted, the heavy conditioner may lead to build-up between the hair tracks and residual will dry white, making the hair look untidy. With leave-in conditioner, you can apply it at least twice a week. To guard against damaged and bad smell from the hair, use braid sheen in adequate amounts.