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 $27.52   12.240%   $30.8884   $30.89   $30.90   $30.90  
  Purchase #2 $103.30   7.648%   $111.2004   $111.20   $111.20   $111.20  
  Purchase #3 $218.44   4.366%   $227.9771   $227.98   $228.00   $228.00  
  Purchase #4 $17.96   4.681%   $18.8007   $18.80   $18.80   $18.80  
  Purchase #5 $159.47   10.465%   $176.1585   $176.16   $176.15   $176.20  
  Purchase #6 $9.55   13.824%   $10.8702   $10.87   $10.85   $10.90  
  Purchase #7 $246.28   6.360%   $261.9434   $261.94   $261.95   $261.90  
  7 purchase totals = $782.52       $837.8387   $837.84   $837.85   $837.90