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 $247.75   3.373%   $256.1066   $256.11   $256.10   $256.10  
  Purchase #2 $167.62   6.161%   $177.9471   $177.95   $177.95   $177.90  
  Purchase #3 $121.67   13.808%   $138.4702   $138.47   $138.45   $138.50  
  Purchase #4 $250.76   4.257%   $261.4349   $261.43   $261.45   $261.40  
  Purchase #5 $45.87   6.727%   $48.9557   $48.96   $48.95   $49.00  
  Purchase #6 $54.42   4.689%   $56.9718   $56.97   $56.95   $57.00  
  Purchase #7 $78.98   3.512%   $81.7538   $81.75   $81.75   $81.80  
  7 purchase totals = $967.07       $1,021.6401   $1,021.64   $1,021.60   $1,021.70