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 $236.53   6.750%   $252.4958   $252.50   $252.50   $252.50  
  Purchase #2 $192.66   3.426%   $199.2605   $199.26   $199.25   $199.30  
  Purchase #3 $60.40   8.411%   $65.4802   $65.48   $65.50   $65.50  
  Purchase #4 $47.63   12.301%   $53.489   $53.49   $53.50   $53.50  
  Purchase #5 $92.60   12.209%   $103.9055   $103.91   $103.90   $103.90  
  Purchase #6 $72.68   11.856%   $81.2969   $81.30   $81.30   $81.30  
  Purchase #7 $76.79   6.805%   $82.0156   $82.02   $82.00   $82.00  
  7 purchase totals = $779.29       $837.9435   $837.96   $837.95   $838.00