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 $220.63   8.265%   $238.8651   $238.87   $238.85   $238.90  
  Purchase #2 $12.74   5.236%   $13.4071   $13.41   $13.40   $13.40  
  Purchase #3 $114.29   7.507%   $122.8698   $122.87   $122.85   $122.90  
  Purchase #4 $21.27   4.109%   $22.144   $22.14   $22.15   $22.10  
  Purchase #5 $159.44   9.393%   $174.4162   $174.42   $174.40   $174.40  
  Purchase #6 $118.68   12.607%   $133.642   $133.64   $133.65   $133.60  
  Purchase #7 $105.77   10.515%   $116.8917   $116.89   $116.90   $116.90  
  7 purchase totals = $752.82       $822.2359   $822.24   $822.20   $822.20