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 $20.32   4.153%   $21.1639   $21.16   $21.15   $21.20  
  Purchase #2 $73.39   10.318%   $80.9624   $80.96   $80.95   $81.00  
  Purchase #3 $237.28   10.317%   $261.7602   $261.76   $261.75   $261.80  
  Purchase #4 $220.26   8.791%   $239.6231   $239.62   $239.60   $239.60  
  Purchase #5 $21.68   4.241%   $22.5994   $22.60   $22.60   $22.60  
  Purchase #6 $244.32   3.603%   $253.1228   $253.12   $253.10   $253.10  
  Purchase #7 $236.72   12.828%   $267.0864   $267.09   $267.10   $267.10  
  7 purchase totals = $1,053.97       $1,146.3182   $1,146.31   $1,146.25   $1,146.40