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 $237.79   4.717%   $249.0066   $249.01   $249.00   $249.00  
  Purchase #2 $70.89   5.515%   $74.7996   $74.80   $74.80   $74.80  
  Purchase #3 $144.27   11.493%   $160.851   $160.85   $160.85   $160.90  
  Purchase #4 $57.92   5.664%   $61.2006   $61.20   $61.20   $61.20  
  Purchase #5 $13.88   12.707%   $15.6437   $15.64   $15.65   $15.60  
  Purchase #6 $245.88   6.807%   $262.6171   $262.62   $262.60   $262.60  
  Purchase #7 $182.76   9.167%   $199.5136   $199.51   $199.50   $199.50  
  7 purchase totals = $953.39       $1,023.6322   $1,023.63   $1,023.60   $1,023.60