I took this introductory passage to be the current tradition. The quoted bible passage makes it look like more of a mandatory thing:

[5] If brethren dwell together, and one of them die, and have no child […]; her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of a husband's brother unto her.

[7] And if the man like not to take his brother's wife, then his brother's wife shall go up to the gate unto the elders, and say: 'My husband's brother refuseth to raise up unto his brother a name in Israel; he will not perform the duty of a husband's brother unto me.'

[9] then shall his brother's wife […] loose his shoe from off his foot, and spit in his face; and […] say: 'So shall it be done unto the man that doth not build up his brother's house.'

Deuteronomy 25 (Modern English)