When is fear good?

Psalm 2:11
Serve the LORD with fear, and rejoice with trembling.
Because God is so great and mighty, and because he holds the power of life and death in his hands, we must have a healthy, reverent fear of him. A healthy fear helps us keep our perspective about where we need to be in our relationship with God.

2 Corinthians 7:1
Having therefore these promises, dearly beloved, let us cleanse ourselves from all filthiness of the flesh and spirit, perfecting holiness in the fear of God.
A healthy fear of God motivates us to strive for holiness.

Deuteronomy 31:7-8
And Moses called unto Joshua, and said unto him in the sight of all Israel, Be strong and of a good courage: for thou must go with this people unto the land which the LORD hath sworn unto their fathers to give them; and thou shalt cause them to inherit it. [8] And the LORD, he it is that doth go before thee; he will be with thee, he will not fail thee, neither forsake thee: fear not, neither be dismayed.
Fear can be good if it teaches us about courage. Joshua couldn’t have truly understood courage if he hadn’t experienced fear. Fear gave him courageous character and taught him to rely on and trust in God.