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 $55.61   6.419%   $59.1796   $59.18   $59.20   $59.20  
  Purchase #2 $206.62   11.703%   $230.8007   $230.80   $230.80   $230.80  
  Purchase #3 $166.66   8.435%   $180.7178   $180.72   $180.70   $180.70  
  Purchase #4 $17.50   3.359%   $18.0878   $18.09   $18.10   $18.10  
  Purchase #5 $67.45   13.411%   $76.4957   $76.50   $76.50   $76.50  
  Purchase #6 $229.76   5.631%   $242.6978   $242.70   $242.70   $242.70  
  Purchase #7 $125.40   4.699%   $131.2925   $131.29   $131.30   $131.30  
  7 purchase totals = $869.00       $939.2719   $939.28   $939.30   $939.30