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 $12.25   5.193%   $12.8861   $12.89   $12.90   $12.90  
  Purchase #2 $229.89   12.107%   $257.7228   $257.72   $257.70   $257.70  
  Purchase #3 $22.78   11.683%   $25.4414   $25.44   $25.45   $25.40  
  Purchase #4 $28.73   9.242%   $31.3852   $31.39   $31.40   $31.40  
  Purchase #5 $209.40   12.106%   $234.75   $234.75   $234.75   $234.80  
  Purchase #6 $66.58   8.865%   $72.4823   $72.48   $72.50   $72.50  
  Purchase #7 $176.89   10.638%   $195.7076   $195.71   $195.70   $195.70  
  7 purchase totals = $746.52       $830.3754   $830.38   $830.40   $830.40