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 $40.58   4.808%   $42.5311   $42.53   $42.55   $42.50  
  Purchase #2 $19.41   5.735%   $20.5232   $20.52   $20.50   $20.50  
  Purchase #3 $121.52   6.562%   $129.4941   $129.49   $129.50   $129.50  
  Purchase #4 $47.96   10.478%   $52.9852   $52.99   $53.00   $53.00  
  Purchase #5 $74.55   7.719%   $80.3045   $80.30   $80.30   $80.30  
  Purchase #6 $126.76   4.869%   $132.9319   $132.93   $132.95   $132.90  
  Purchase #7 $73.36   11.399%   $81.7223   $81.72   $81.70   $81.70  
  7 purchase totals = $504.14       $540.4923   $540.48   $540.50   $540.40