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 $249.53   13.475%   $283.1542   $283.15   $283.15   $283.20  
  Purchase #2 $113.81   11.510%   $126.9095   $126.91   $126.90   $126.90  
  Purchase #3 $244.49   8.610%   $265.5406   $265.54   $265.55   $265.50  
  Purchase #4 $106.82   4.474%   $111.5991   $111.60   $111.60   $111.60  
  Purchase #5 $136.90   3.804%   $142.1077   $142.11   $142.10   $142.10  
  Purchase #6 $122.98   10.162%   $135.4772   $135.48   $135.50   $135.50  
  Purchase #7 $163.46   11.166%   $181.7119   $181.71   $181.70   $181.70  
  7 purchase totals = $1,137.99       $1,246.5002   $1,246.50   $1,246.50   $1,246.50