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 $244.80   13.805%   $278.5946   $278.59   $278.60   $278.60  
  Purchase #2 $226.71   11.826%   $253.5207   $253.52   $253.50   $253.50  
  Purchase #3 $131.61   4.277%   $137.239   $137.24   $137.25   $137.20  
  Purchase #4 $223.80   12.493%   $251.7593   $251.76   $251.75   $251.80  
  Purchase #5 $32.48   12.192%   $36.44   $36.44   $36.45   $36.40  
  Purchase #6 $177.91   3.785%   $184.6439   $184.64   $184.65   $184.60  
  Purchase #7 $123.73   3.725%   $128.3389   $128.34   $128.35   $128.30  
  7 purchase totals = $1,161.04       $1,270.5364   $1,270.53   $1,270.55   $1,270.40