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 $249.80   13.548%   $283.6429   $283.64   $283.65   $283.60  
  Purchase #2 $217.87   4.617%   $227.9291   $227.93   $227.95   $227.90  
  Purchase #3 $248.70   7.211%   $266.6338   $266.63   $266.65   $266.60  
  Purchase #4 $245.34   7.249%   $263.1247   $263.12   $263.10   $263.10  
  Purchase #5 $143.37   13.503%   $162.7293   $162.73   $162.75   $162.70  
  Purchase #6 $204.49   10.725%   $226.4216   $226.42   $226.40   $226.40  
  Purchase #7 $231.45   10.246%   $255.1644   $255.16   $255.15   $255.20  
  7 purchase totals = $1,541.02       $1,685.6458   $1,685.63   $1,685.65   $1,685.50