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 $28.75   7.796%   $30.9914   $30.99   $31.00   $31.00  
  Purchase #2 $147.40   8.855%   $160.4523   $160.45   $160.45   $160.50  
  Purchase #3 $76.43   13.831%   $87.001   $87.00   $87.00   $87.00  
  Purchase #4 $176.68   13.422%   $200.394   $200.39   $200.40   $200.40  
  Purchase #5 $163.81   8.192%   $177.2293   $177.23   $177.25   $177.20  
  Purchase #6 $180.89   9.244%   $197.6115   $197.61   $197.60   $197.60  
  Purchase #7 $128.83   6.497%   $137.2001   $137.20   $137.20   $137.20  
  7 purchase totals = $902.79       $990.8796   $990.87   $990.90   $990.90