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 $11.33   10.544%   $12.5246   $12.52   $12.50   $12.50  
  Purchase #2 $137.34   11.744%   $153.4692   $153.47   $153.45   $153.50  
  Purchase #3 $26.55   5.272%   $27.9497   $27.95   $27.95   $27.90  
  Purchase #4 $55.26   9.334%   $60.418   $60.42   $60.40   $60.40  
  Purchase #5 $55.65   9.385%   $60.8728   $60.87   $60.85   $60.90  
  Purchase #6 $219.78   13.734%   $249.9646   $249.96   $249.95   $250.00  
  Purchase #7 $190.53   12.578%   $214.4949   $214.49   $214.50   $214.50  
  7 purchase totals = $696.44       $779.6938   $779.68   $779.60   $779.70