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 $176.53   8.314%   $191.2067   $191.21   $191.20   $191.20  
  Purchase #2 $46.43   7.169%   $49.7586   $49.76   $49.75   $49.80  
  Purchase #3 $59.52   11.233%   $66.2059   $66.21   $66.20   $66.20  
  Purchase #4 $114.26   4.506%   $119.4086   $119.41   $119.40   $119.40  
  Purchase #5 $118.71   13.496%   $134.7311   $134.73   $134.75   $134.70  
  Purchase #6 $43.44   10.822%   $48.1411   $48.14   $48.15   $48.10  
  Purchase #7 $67.40   4.643%   $70.5294   $70.53   $70.55   $70.50  
  7 purchase totals = $626.29       $679.9814   $679.99   $680.00   $679.90