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 $54.47   13.289%   $61.7085   $61.71   $61.70   $61.70  
  Purchase #2 $111.96   7.726%   $120.61   $120.61   $120.60   $120.60  
  Purchase #3 $35.35   3.612%   $36.6268   $36.63   $36.65   $36.60  
  Purchase #4 $234.54   13.133%   $265.3421   $265.34   $265.35   $265.30  
  Purchase #5 $75.44   11.114%   $83.8244   $83.82   $83.80   $83.80  
  Purchase #6 $117.29   7.514%   $126.1032   $126.10   $126.10   $126.10  
  Purchase #7 $134.70   8.673%   $146.3825   $146.38   $146.40   $146.40  
  7 purchase totals = $763.75       $840.5975   $840.59   $840.60   $840.50