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 $19.57   9.255%   $21.3812   $21.38   $21.40   $21.40  
  Purchase #2 $37.88   3.678%   $39.2732   $39.27   $39.25   $39.30  
  Purchase #3 $51.77   4.413%   $54.0546   $54.05   $54.05   $54.10  
  Purchase #4 $103.62   5.404%   $109.2196   $109.22   $109.20   $109.20  
  Purchase #5 $209.76   12.588%   $236.1646   $236.16   $236.15   $236.20  
  Purchase #6 $171.37   5.587%   $180.9444   $180.94   $180.95   $180.90  
  Purchase #7 $219.87   13.206%   $248.906   $248.91   $248.90   $248.90  
  7 purchase totals = $813.84       $889.9436   $889.93   $889.90   $890.00