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 $21.55   4.585%   $22.5381   $22.54   $22.55   $22.50  
  Purchase #2 $70.71   4.717%   $74.0454   $74.05   $74.05   $74.00  
  Purchase #3 $121.75   13.723%   $138.4578   $138.46   $138.45   $138.50  
  Purchase #4 $213.78   8.251%   $231.419   $231.42   $231.40   $231.40  
  Purchase #5 $250.35   8.824%   $272.4409   $272.44   $272.45   $272.40  
  Purchase #6 $79.63   10.542%   $88.0246   $88.02   $88.00   $88.00  
  Purchase #7 $144.25   9.361%   $157.7532   $157.75   $157.75   $157.80  
  7 purchase totals = $902.02       $984.6790   $984.68   $984.65   $984.60