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 $19.89   9.434%   $21.7664   $21.77   $21.75   $21.80  
  Purchase #2 $166.88   4.142%   $173.7922   $173.79   $173.80   $173.80  
  Purchase #3 $197.91   6.498%   $210.7702   $210.77   $210.75   $210.80  
  Purchase #4 $26.72   7.352%   $28.6845   $28.68   $28.70   $28.70  
  Purchase #5 $29.44   11.395%   $32.7947   $32.79   $32.80   $32.80  
  Purchase #6 $203.43   12.691%   $229.2473   $229.25   $229.25   $229.20  
  Purchase #7 $112.82   11.639%   $125.9511   $125.95   $125.95   $126.00  
  7 purchase totals = $757.09       $823.0064   $823.00   $823.00   $823.10