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 $21.49   12.434%   $24.1621   $24.16   $24.15   $24.20  
  Purchase #2 $99.56   7.106%   $106.6347   $106.63   $106.65   $106.60  
  Purchase #3 $113.94   6.528%   $121.378   $121.38   $121.40   $121.40  
  Purchase #4 $235.27   5.400%   $247.9746   $247.97   $247.95   $248.00  
  Purchase #5 $90.72   6.781%   $96.8717   $96.87   $96.85   $96.90  
  Purchase #6 $165.66   8.281%   $179.3783   $179.38   $179.40   $179.40  
  Purchase #7 $158.59   10.534%   $175.2959   $175.30   $175.30   $175.30  
  7 purchase totals = $885.23       $951.6953   $951.69   $951.70   $951.80