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 $20.46   4.555%   $21.392   $21.39   $21.40   $21.40  
  Purchase #2 $199.59   9.789%   $219.1279   $219.13   $219.15   $219.10  
  Purchase #3 $217.51   3.625%   $225.3947   $225.39   $225.40   $225.40  
  Purchase #4 $127.93   5.337%   $134.7576   $134.76   $134.75   $134.80  
  Purchase #5 $70.75   11.857%   $79.1388   $79.14   $79.15   $79.10  
  Purchase #6 $172.51   10.311%   $190.2975   $190.30   $190.30   $190.30  
  Purchase #7 $19.80   5.780%   $20.9444   $20.94   $20.95   $20.90  
  7 purchase totals = $828.55       $891.0529   $891.05   $891.10   $891.00