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 $224.98   10.687%   $249.0236   $249.02   $249.00   $249.00  
  Purchase #2 $161.27   6.370%   $171.5429   $171.54   $171.55   $171.50  
  Purchase #3 $122.63   7.227%   $131.4925   $131.49   $131.50   $131.50  
  Purchase #4 $197.33   4.381%   $205.975   $205.98   $206.00   $206.00  
  Purchase #5 $226.45   3.293%   $233.907   $233.91   $233.90   $233.90  
  Purchase #6 $214.33   10.837%   $237.5569   $237.56   $237.55   $237.60  
  Purchase #7 $109.73   3.381%   $113.44   $113.44   $113.45   $113.40  
  7 purchase totals = $1,256.72       $1,342.9379   $1,342.94   $1,342.95   $1,342.90