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 $19.74   12.387%   $22.1852   $22.19   $22.20   $22.20  
  Purchase #2 $185.54   10.208%   $204.4799   $204.48   $204.50   $204.50  
  Purchase #3 $100.61   4.273%   $104.9091   $104.91   $104.90   $104.90  
  Purchase #4 $148.43   6.337%   $157.836   $157.84   $157.85   $157.80  
  Purchase #5 $98.84   13.522%   $112.2051   $112.21   $112.20   $112.20  
  Purchase #6 $9.42   12.692%   $10.6156   $10.62   $10.60   $10.60  
  Purchase #7 $22.30   11.679%   $24.9044   $24.90   $24.90   $24.90  
  7 purchase totals = $584.88       $637.1353   $637.15   $637.15   $637.10