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 $15.68   11.809%   $17.5317   $17.53   $17.55   $17.50  
  Purchase #2 $12.91   5.786%   $13.657   $13.66   $13.65   $13.70  
  Purchase #3 $125.81   8.720%   $136.7806   $136.78   $136.80   $136.80  
  Purchase #4 $110.58   9.572%   $121.1647   $121.16   $121.15   $121.20  
  Purchase #5 $11.50   3.346%   $11.8848   $11.88   $11.90   $11.90  
  Purchase #6 $70.48   9.195%   $76.9606   $76.96   $76.95   $77.00  
  Purchase #7 $89.71   12.532%   $100.9525   $100.95   $100.95   $101.00  
  7 purchase totals = $436.67       $478.9319   $478.92   $478.95   $479.10