When it comes to genetic engineering when does enhancement tip over into genetic corruption?

A most difficult question to answer. 

When we get to the point where we start manufacturing children to meet our requirements we'll most certainly have gone too far. BUT, suppose a child could be saved from a hereditary ailment, would that be OK. 

The line would need to be drawn very carefully.

The problem with genetic engineering is "unintended consequences", which is a polite way of saying they don't know what they're doing but it's not their fault.