Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $52.63   9.238%   $57.492   $57.49   $57.50   $57.50  
  Purchase #2 $29.46   6.298%   $31.3154   $31.32   $31.30   $31.30  
  Purchase #3 $158.73   7.192%   $170.1459   $170.15   $170.15   $170.10  
  Purchase #4 $157.71   3.691%   $163.5311   $163.53   $163.55   $163.50  
  Purchase #5 $100.36   10.499%   $110.8968   $110.90   $110.90   $110.90  
  Purchase #6 $28.36   4.103%   $29.5236   $29.52   $29.50   $29.50  
  Purchase #7 $73.51   4.846%   $77.0723   $77.07   $77.05   $77.10  
  7 purchase totals = $600.76       $639.9771   $639.98   $639.95   $639.90