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 $249.99   10.382%   $275.944   $275.94   $275.95   $275.90  
  Purchase #2 $250.41   9.646%   $274.5645   $274.56   $274.55   $274.60  
  Purchase #3 $13.73   12.157%   $15.3992   $15.40   $15.40   $15.40  
  Purchase #4 $177.58   7.715%   $191.2803   $191.28   $191.30   $191.30  
  Purchase #5 $92.54   4.584%   $96.782   $96.78   $96.80   $96.80  
  Purchase #6 $11.25   3.378%   $11.63   $11.63   $11.65   $11.60  
  Purchase #7 $157.53   12.207%   $176.7597   $176.76   $176.75   $176.80  
  7 purchase totals = $953.03       $1,042.3597   $1,042.35   $1,042.40   $1,042.40