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 $68.63   11.235%   $76.3406   $76.34   $76.35   $76.30  
  Purchase #2 $15.32   4.476%   $16.0057   $16.01   $16.00   $16.00  
  Purchase #3 $176.56   5.778%   $186.7616   $186.76   $186.75   $186.80  
  Purchase #4 $32.92   12.269%   $36.959   $36.96   $36.95   $37.00  
  Purchase #5 $123.30   10.461%   $136.1984   $136.20   $136.20   $136.20  
  Purchase #6 $60.59   12.712%   $68.2922   $68.29   $68.30   $68.30  
  Purchase #7 $75.29   10.760%   $83.3912   $83.39   $83.40   $83.40  
  7 purchase totals = $552.61       $603.9487   $603.95   $603.95   $604.00