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 $17.99   4.118%   $18.7308   $18.73   $18.75   $18.70  
  Purchase #2 $107.49   3.327%   $111.0662   $111.07   $111.05   $111.10  
  Purchase #3 $206.41   7.383%   $221.6493   $221.65   $221.65   $221.60  
  Purchase #4 $7.29   12.178%   $8.1778   $8.18   $8.20   $8.20  
  Purchase #5 $27.88   10.459%   $30.796   $30.80   $30.80   $30.80  
  Purchase #6 $64.25   5.120%   $67.5396   $67.54   $67.55   $67.50  
  Purchase #7 $13.84   13.570%   $15.7181   $15.72   $15.70   $15.70  
  7 purchase totals = $445.15       $473.6778   $473.69   $473.70   $473.60