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 $16.78   12.397%   $18.8602   $18.86   $18.85   $18.90  
  Purchase #2 $117.60   11.201%   $130.7724   $130.77   $130.75   $130.80  
  Purchase #3 $214.68   10.519%   $237.2622   $237.26   $237.25   $237.30  
  Purchase #4 $61.25   11.628%   $68.3722   $68.37   $68.35   $68.40  
  Purchase #5 $20.59   13.509%   $23.3715   $23.37   $23.35   $23.40  
  Purchase #6 $134.55   5.820%   $142.3808   $142.38   $142.40   $142.40  
  Purchase #7 $241.81   6.434%   $257.3681   $257.37   $257.35   $257.40  
  7 purchase totals = $807.26       $878.3874   $878.38   $878.30   $878.60