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 $19.97   6.300%   $21.2281   $21.23   $21.25   $21.20  
  Purchase #2 $169.49   13.568%   $192.4864   $192.49   $192.50   $192.50  
  Purchase #3 $109.56   13.268%   $124.0964   $124.10   $124.10   $124.10  
  Purchase #4 $163.88   3.775%   $170.0665   $170.07   $170.05   $170.10  
  Purchase #5 $10.83   6.510%   $11.535   $11.54   $11.55   $11.50  
  Purchase #6 $72.84   7.587%   $78.3664   $78.37   $78.35   $78.40  
  Purchase #7 $106.64   10.371%   $117.6996   $117.70   $117.70   $117.70  
  7 purchase totals = $653.21       $715.4784   $715.50   $715.50   $715.50