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 $15.70   13.777%   $17.863   $17.86   $17.85   $17.90  
  Purchase #2 $243.40   11.331%   $270.9797   $270.98   $271.00   $271.00  
  Purchase #3 $50.77   8.858%   $55.2672   $55.27   $55.25   $55.30  
  Purchase #4 $25.43   13.139%   $28.7712   $28.77   $28.75   $28.80  
  Purchase #5 $29.78   4.427%   $31.0984   $31.10   $31.10   $31.10  
  Purchase #6 $133.59   5.825%   $141.3716   $141.37   $141.35   $141.40  
  Purchase #7 $53.67   12.534%   $60.397   $60.40   $60.40   $60.40  
  7 purchase totals = $552.34       $605.7481   $605.75   $605.70   $605.90