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 $248.74   11.169%   $276.5218   $276.52   $276.50   $276.50  
  Purchase #2 $31.74   13.186%   $35.9252   $35.93   $35.95   $35.90  
  Purchase #3 $244.84   6.466%   $260.6714   $260.67   $260.65   $260.70  
  Purchase #4 $200.52   7.535%   $215.6292   $215.63   $215.65   $215.60  
  Purchase #5 $224.76   9.849%   $246.8966   $246.90   $246.90   $246.90  
  Purchase #6 $197.42   13.467%   $224.0066   $224.01   $224.00   $224.00  
  Purchase #7 $6.33   5.215%   $6.6601   $6.66   $6.65   $6.70  
  7 purchase totals = $1,154.35       $1,266.3109   $1,266.32   $1,266.30   $1,266.30