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 $19.28   8.216%   $20.864   $20.86   $20.85   $20.90  
  Purchase #2 $17.55   13.838%   $19.9786   $19.98   $20.00   $20.00  
  Purchase #3 $234.57   6.597%   $250.0446   $250.04   $250.05   $250.00  
  Purchase #4 $49.99   3.531%   $51.7551   $51.76   $51.75   $51.80  
  Purchase #5 $129.86   11.204%   $144.4095   $144.41   $144.40   $144.40  
  Purchase #6 $206.70   12.682%   $232.9137   $232.91   $232.90   $232.90  
  Purchase #7 $94.71   13.857%   $107.834   $107.83   $107.85   $107.80  
  7 purchase totals = $752.66       $827.7995   $827.79   $827.80   $827.80