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 $18.77   12.700%   $21.1538   $21.15   $21.15   $21.20  
  Purchase #2 $82.99   7.454%   $89.1761   $89.18   $89.20   $89.20  
  Purchase #3 $23.47   3.807%   $24.3635   $24.36   $24.35   $24.40  
  Purchase #4 $14.25   11.836%   $15.9366   $15.94   $15.95   $15.90  
  Purchase #5 $44.75   11.204%   $49.7638   $49.76   $49.75   $49.80  
  Purchase #6 $140.34   12.749%   $158.2319   $158.23   $158.25   $158.20  
  Purchase #7 $242.97   6.837%   $259.5819   $259.58   $259.60   $259.60  
  7 purchase totals = $567.54       $618.2076   $618.20   $618.25   $618.30