Every individual betta has its own character and habits. Some are psychotic egg/fry eaters while others are good parents. And occasionally a once good parent may turn psychotic and eat his batch. So which method to use really depends on the male's character. Usually if he takes care of his eggs, he should care for his fry - unless disturbed too much.
I now believe that the male somehow feeds his fry (if left with fry). This may explain why fry usually grows faster when daddy's around. BUT if the spawn is very important to you, it is safer to take daddy out once fry are free swimming.