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 $20.40   3.361%   $21.0856   $21.09   $21.10   $21.10  
  Purchase #2 $131.61   4.650%   $137.7299   $137.73   $137.75   $137.70  
  Purchase #3 $217.57   6.596%   $231.9209   $231.92   $231.90   $231.90  
  Purchase #4 $185.53   3.243%   $191.5467   $191.55   $191.55   $191.50  
  Purchase #5 $219.27   9.347%   $239.7652   $239.77   $239.75   $239.80  
  Purchase #6 $34.91   6.493%   $37.1767   $37.18   $37.20   $37.20  
  Purchase #7 $74.42   6.205%   $79.0378   $79.04   $79.05   $79.00  
  7 purchase totals = $883.71       $938.2628   $938.28   $938.30   $938.20