Iowa Admin. Code r. 875-217.57
The employee must receive more than $30 a month in tips "customarily and regularly" in the occupation in which the employee is engaged in order to qualify as a tipped employee. If it is known that the employee always receives more than the stipulated amount each month, as may be the case with many employees in occupations such as those of food server, bellhop, taxicab driver, barber, or beautician, the employee will qualify and the tip credit provisions may be applied. Alternatively, an employee who only occasionally or sporadically receives tips totaling more than $30 a month, such as at holidays when customers may be more generous than usual, will not be deemed a tipped employee. The phrase "customarily and regularly" signifies a frequency which must be greater than occasional, but which may be less than constant. If an employee is in an occupation in which the employee normally and recurrently receives more than $30 a month in tips, the employee will be considered a tipped employee even though occasionally because of sickness, vacation, seasonal fluctuations or the like, the employee fails to receive more than $30 in tips in a particular month.
Iowa Admin. Code r. 875-217.57
29 CFR 531.57.