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 $7.81   13.547%   $8.868   $8.87   $8.85   $8.90  
  Purchase #2 $28.30   7.550%   $30.4367   $30.44   $30.45   $30.40  
  Purchase #3 $222.63   9.745%   $244.3253   $244.33   $244.35   $244.30  
  Purchase #4 $47.93   8.238%   $51.8785   $51.88   $51.90   $51.90  
  Purchase #5 $247.58   5.360%   $260.8503   $260.85   $260.85   $260.90  
  Purchase #6 $11.82   13.179%   $13.3778   $13.38   $13.40   $13.40  
  Purchase #7 $67.63   6.661%   $72.1348   $72.13   $72.15   $72.10  
  7 purchase totals = $633.70       $681.8714   $681.88   $681.95   $681.90