I don't know why you'd ever think not holding grudges is unhealthy when there's tons of places in society that tell you grudges are unhealthy. The only problem you have to watch out for which isn't directly related (nor cause to try and hold grudges) is getting repeatedly abused. Grudges are an unhealthy way to protect ourselves from being hurt again. You can avoid being abused again without grudges, but it's a more conscious effort.