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 $227.40   8.274%   $246.2151   $246.22   $246.20   $246.20  
  Purchase #2 $181.53   8.103%   $196.2394   $196.24   $196.25   $196.20  
  Purchase #3 $58.32   13.495%   $66.1903   $66.19   $66.20   $66.20  
  Purchase #4 $139.52   11.744%   $155.9052   $155.91   $155.90   $155.90  
  Purchase #5 $85.39   3.471%   $88.3539   $88.35   $88.35   $88.40  
  Purchase #6 $36.34   8.265%   $39.3435   $39.34   $39.35   $39.30  
  Purchase #7 $157.31   4.240%   $163.9799   $163.98   $164.00   $164.00  
  7 purchase totals = $885.81       $956.2273   $956.23   $956.25   $956.20