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 $56.51   6.105%   $59.9599   $59.96   $59.95   $60.00  
  Purchase #2 $9.92   9.468%   $10.8592   $10.86   $10.85   $10.90  
  Purchase #3 $50.66   5.262%   $53.3257   $53.33   $53.35   $53.30  
  Purchase #4 $84.83   7.567%   $91.2491   $91.25   $91.25   $91.20  
  Purchase #5 $63.55   6.428%   $67.635   $67.64   $67.65   $67.60  
  Purchase #6 $125.30   3.743%   $129.99   $129.99   $130.00   $130.00  
  Purchase #7 $162.93   10.808%   $180.5395   $180.54   $180.55   $180.50  
  7 purchase totals = $553.70       $593.5584   $593.57   $593.60   $593.50