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 $20.41   13.646%   $23.1951   $23.20   $23.20   $23.20  
  Purchase #2 $114.86   5.397%   $121.059   $121.06   $121.05   $121.10  
  Purchase #3 $128.39   11.501%   $143.1561   $143.16   $143.15   $143.20  
  Purchase #4 $40.84   11.573%   $45.5664   $45.57   $45.55   $45.60  
  Purchase #5 $103.87   3.508%   $107.5138   $107.51   $107.50   $107.50  
  Purchase #6 $32.82   11.791%   $36.6898   $36.69   $36.70   $36.70  
  Purchase #7 $30.98   5.274%   $32.6139   $32.61   $32.60   $32.60  
  7 purchase totals = $472.17       $509.7941   $509.80   $509.75   $509.90