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 $241.73   5.492%   $255.0058   $255.01   $255.00   $255.00  
  Purchase #2 $105.69   10.296%   $116.5718   $116.57   $116.55   $116.60  
  Purchase #3 $232.56   7.409%   $249.7904   $249.79   $249.80   $249.80  
  Purchase #4 $84.75   12.824%   $95.6183   $95.62   $95.60   $95.60  
  Purchase #5 $123.61   6.725%   $131.9228   $131.92   $131.90   $131.90  
  Purchase #6 $122.93   12.597%   $138.4155   $138.42   $138.40   $138.40  
  Purchase #7 $218.94   9.212%   $239.1088   $239.11   $239.10   $239.10  
  7 purchase totals = $1,130.21       $1,226.4334   $1,226.44   $1,226.35   $1,226.40