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 $248.67   9.529%   $272.3658   $272.37   $272.35   $272.40  
  Purchase #2 $21.49   11.550%   $23.9721   $23.97   $23.95   $24.00  
  Purchase #3 $114.92   8.169%   $124.3078   $124.31   $124.30   $124.30  
  Purchase #4 $76.98   7.463%   $82.725   $82.73   $82.75   $82.70  
  Purchase #5 $117.55   8.532%   $127.5794   $127.58   $127.60   $127.60  
  Purchase #6 $150.49   5.504%   $158.773   $158.77   $158.75   $158.80  
  Purchase #7 $55.99   6.232%   $59.4793   $59.48   $59.50   $59.50  
  7 purchase totals = $786.09       $849.2024   $849.21   $849.20   $849.30