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 $18.30   9.398%   $20.0198   $20.02   $20.00   $20.00  
  Purchase #2 $158.82   11.181%   $176.5777   $176.58   $176.60   $176.60  
  Purchase #3 $112.64   12.111%   $126.2818   $126.28   $126.30   $126.30  
  Purchase #4 $11.61   6.792%   $12.3986   $12.40   $12.40   $12.40  
  Purchase #5 $66.81   11.562%   $74.5346   $74.53   $74.55   $74.50  
  Purchase #6 $51.97   6.507%   $55.3517   $55.35   $55.35   $55.40  
  Purchase #7 $115.41   11.768%   $128.9914   $128.99   $129.00   $129.00  
  7 purchase totals = $535.56       $594.1556   $594.15   $594.20   $594.20