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 $19.99   10.386%   $22.0662   $22.07   $22.05   $22.10  
  Purchase #2 $218.28   12.730%   $246.067   $246.07   $246.05   $246.10  
  Purchase #3 $142.37   12.266%   $159.8331   $159.83   $159.85   $159.80  
  Purchase #4 $42.72   3.112%   $44.0494   $44.05   $44.05   $44.00  
  Purchase #5 $121.88   3.281%   $125.8789   $125.88   $125.90   $125.90  
  Purchase #6 $107.76   5.541%   $113.731   $113.73   $113.75   $113.70  
  Purchase #7 $94.50   5.434%   $99.6351   $99.64   $99.65   $99.60  
  7 purchase totals = $747.50       $811.2607   $811.27   $811.30   $811.20