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 $16.99   4.831%   $17.8108   $17.81   $17.80   $17.80  
  Purchase #2 $179.96   5.697%   $190.2123   $190.21   $190.20   $190.20  
  Purchase #3 $198.83   13.278%   $225.2306   $225.23   $225.25   $225.20  
  Purchase #4 $109.64   9.546%   $120.1062   $120.11   $120.10   $120.10  
  Purchase #5 $105.76   13.848%   $120.4056   $120.41   $120.40   $120.40  
  Purchase #6 $79.51   8.107%   $85.9559   $85.96   $85.95   $86.00  
  Purchase #7 $196.38   8.788%   $213.6379   $213.64   $213.65   $213.60  
  7 purchase totals = $887.07       $973.3593   $973.37   $973.35   $973.30