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 $18.75   12.771%   $21.1446   $21.14   $21.15   $21.10  
  Purchase #2 $171.46   8.570%   $186.1541   $186.15   $186.15   $186.20  
  Purchase #3 $131.68   9.535%   $144.2357   $144.24   $144.25   $144.20  
  Purchase #4 $101.73   10.498%   $112.4096   $112.41   $112.40   $112.40  
  Purchase #5 $47.90   10.411%   $52.8869   $52.89   $52.90   $52.90  
  Purchase #6 $197.98   6.821%   $211.4842   $211.48   $211.50   $211.50  
  Purchase #7 $113.73   13.636%   $129.2382   $129.24   $129.25   $129.20  
  7 purchase totals = $783.23       $857.5533   $857.55   $857.60   $857.50