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 $31.44   4.368%   $32.8133   $32.81   $32.80   $32.80  
  Purchase #2 $20.27   3.237%   $20.9261   $20.93   $20.95   $20.90  
  Purchase #3 $71.98   10.422%   $79.4818   $79.48   $79.50   $79.50  
  Purchase #4 $183.61   6.282%   $195.1444   $195.14   $195.15   $195.10  
  Purchase #5 $25.57   8.358%   $27.7071   $27.71   $27.70   $27.70  
  Purchase #6 $140.38   5.473%   $148.063   $148.06   $148.05   $148.10  
  Purchase #7 $42.60   5.866%   $45.0989   $45.10   $45.10   $45.10  
  7 purchase totals = $515.85       $549.2346   $549.23   $549.25   $549.20