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 $229.74   13.611%   $261.0099   $261.01   $261.00   $261.00  
  Purchase #2 $11.41   6.615%   $12.1648   $12.16   $12.15   $12.20  
  Purchase #3 $114.43   10.660%   $126.6282   $126.63   $126.65   $126.60  
  Purchase #4 $131.68   9.577%   $144.291   $144.29   $144.30   $144.30  
  Purchase #5 $245.41   12.476%   $276.0274   $276.03   $276.05   $276.00  
  Purchase #6 $211.44   7.666%   $227.649   $227.65   $227.65   $227.60  
  Purchase #7 $47.98   5.237%   $50.4927   $50.49   $50.50   $50.50  
  7 purchase totals = $992.09       $1,098.2630   $1,098.26   $1,098.30   $1,098.20