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 $30.30   3.184%   $31.2648   $31.26   $31.25   $31.30  
  Purchase #2 $153.38   6.639%   $163.5629   $163.56   $163.55   $163.60  
  Purchase #3 $96.82   8.819%   $105.3586   $105.36   $105.35   $105.40  
  Purchase #4 $12.58   8.868%   $13.6956   $13.70   $13.70   $13.70  
  Purchase #5 $176.62   12.766%   $199.1673   $199.17   $199.15   $199.20  
  Purchase #6 $130.35   5.644%   $137.707   $137.71   $137.70   $137.70  
  Purchase #7 $146.73   7.211%   $157.3107   $157.31   $157.30   $157.30  
  7 purchase totals = $746.78       $808.0669   $808.07   $808.00   $808.20