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 $211.30   13.276%   $239.3522   $239.35   $239.35   $239.40  
  Purchase #2 $223.65   11.632%   $249.665   $249.67   $249.65   $249.70  
  Purchase #3 $53.88   12.244%   $60.4771   $60.48   $60.50   $60.50  
  Purchase #4 $86.85   12.734%   $97.9095   $97.91   $97.90   $97.90  
  Purchase #5 $175.55   9.398%   $192.0482   $192.05   $192.05   $192.00  
  Purchase #6 $21.83   12.760%   $24.6155   $24.62   $24.60   $24.60  
  Purchase #7 $51.47   5.709%   $54.4084   $54.41   $54.40   $54.40  
  7 purchase totals = $824.53       $918.4759   $918.49   $918.45   $918.50