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 $19.72   6.439%   $20.9898   $20.99   $21.00   $21.00  
  Purchase #2 $209.61   10.660%   $231.9544   $231.95   $231.95   $232.00  
  Purchase #3 $75.83   7.615%   $81.6045   $81.60   $81.60   $81.60  
  Purchase #4 $81.59   8.134%   $88.2265   $88.23   $88.25   $88.20  
  Purchase #5 $121.90   4.756%   $127.6976   $127.70   $127.70   $127.70  
  Purchase #6 $96.53   6.689%   $102.9869   $102.99   $103.00   $103.00  
  Purchase #7 $11.25   8.363%   $12.1908   $12.19   $12.20   $12.20  
  7 purchase totals = $616.43       $665.6505   $665.65   $665.70   $665.70