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 $19.80   6.728%   $21.1321   $21.13   $21.15   $21.10  
  Purchase #2 $37.76   5.368%   $39.787   $39.79   $39.80   $39.80  
  Purchase #3 $248.63   5.212%   $261.5886   $261.59   $261.60   $261.60  
  Purchase #4 $64.51   12.359%   $72.4828   $72.48   $72.50   $72.50  
  Purchase #5 $199.79   8.346%   $216.4645   $216.46   $216.45   $216.50  
  Purchase #6 $78.81   8.209%   $85.2795   $85.28   $85.30   $85.30  
  Purchase #7 $9.69   3.345%   $10.0141   $10.01   $10.00   $10.00  
  7 purchase totals = $658.99       $706.7486   $706.74   $706.80   $706.80