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 $183.33   12.499%   $206.2444   $206.24   $206.25   $206.20  
  Purchase #2 $22.32   5.117%   $23.4621   $23.46   $23.45   $23.50  
  Purchase #3 $234.26   4.274%   $244.2723   $244.27   $244.25   $244.30  
  Purchase #4 $88.93   5.532%   $93.8496   $93.85   $93.85   $93.80  
  Purchase #5 $130.96   13.726%   $148.9356   $148.94   $148.95   $148.90  
  Purchase #6 $241.36   3.505%   $249.8197   $249.82   $249.80   $249.80  
  Purchase #7 $249.32   4.240%   $259.8912   $259.89   $259.90   $259.90  
  7 purchase totals = $1,150.48       $1,226.4749   $1,226.47   $1,226.45   $1,226.40