Leviticus 19:27

Ye shall not round the corners of your heads, neither shalt thou mar the corners of thy beard.

Matthew 5:19-20

19 Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.
20 For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven.

It's ok to trim down but don't mar it, meaning shave it off any where, this includes shaping it by balding it in areas hair grows.
The same is true with hair cuts, we can trim it low but we are not to round the corners of our heads meaning, not to bald our hair on our heads where hair grows.

Deuteronomy 14:1

Ye are the children of the Lord your God: ye shall not cut yourselves, nor make any baldness between your eyes for the dead.

Distinguishing Israel from pagans: These laws were intended to set the Israelites apart from the idolatrous practices of the heathens, who would gash themselves or shave specific parts of their heads and faces to honor the dead or their gods.

Unlawful To Shave Your Head And Facial Hair