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 $25.76   7.446%   $27.6781   $27.68   $27.70   $27.70  
  Purchase #2 $134.99   3.420%   $139.6067   $139.61   $139.60   $139.60  
  Purchase #3 $131.84   11.818%   $147.4209   $147.42   $147.40   $147.40  
  Purchase #4 $84.59   13.795%   $96.2592   $96.26   $96.25   $96.30  
  Purchase #5 $164.83   7.527%   $177.2368   $177.24   $177.25   $177.20  
  Purchase #6 $76.68   10.544%   $84.7651   $84.77   $84.75   $84.80  
  Purchase #7 $5.80   10.403%   $6.4034   $6.40   $6.40   $6.40  
  7 purchase totals = $624.49       $679.3702   $679.38   $679.35   $679.40