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 $28.90   7.509%   $31.0701   $31.07   $31.05   $31.10  
  Purchase #2 $31.48   8.457%   $34.1423   $34.14   $34.15   $34.10  
  Purchase #3 $146.89   4.451%   $153.4281   $153.43   $153.45   $153.40  
  Purchase #4 $109.42   7.577%   $117.7108   $117.71   $117.70   $117.70  
  Purchase #5 $10.41   8.602%   $11.3055   $11.31   $11.30   $11.30  
  Purchase #6 $245.99   8.319%   $266.4539   $266.45   $266.45   $266.50  
  Purchase #7 $45.96   8.631%   $49.9268   $49.93   $49.95   $49.90  
  7 purchase totals = $619.05       $664.0375   $664.04   $664.05   $664.00