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 $20.85   4.538%   $21.7962   $21.80   $21.80   $21.80  
  Purchase #2 $70.70   10.761%   $78.308   $78.31   $78.30   $78.30  
  Purchase #3 $38.50   4.448%   $40.2125   $40.21   $40.20   $40.20  
  Purchase #4 $54.83   9.460%   $60.0169   $60.02   $60.00   $60.00  
  Purchase #5 $171.70   7.274%   $184.1895   $184.19   $184.20   $184.20  
  Purchase #6 $26.97   3.120%   $27.8115   $27.81   $27.80   $27.80  
  Purchase #7 $113.83   4.669%   $119.1447   $119.14   $119.15   $119.10  
  7 purchase totals = $497.38       $531.4793   $531.48   $531.45   $531.40