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 $6.43   13.347%   $7.2882   $7.29   $7.30   $7.30  
  Purchase #2 $17.99   8.757%   $19.5654   $19.57   $19.55   $19.60  
  Purchase #3 $198.95   7.750%   $214.3686   $214.37   $214.35   $214.40  
  Purchase #4 $84.82   12.520%   $95.4395   $95.44   $95.45   $95.40  
  Purchase #5 $120.87   8.301%   $130.9034   $130.90   $130.90   $130.90  
  Purchase #6 $64.66   5.806%   $68.4142   $68.41   $68.40   $68.40  
  Purchase #7 $144.44   8.575%   $156.8257   $156.83   $156.85   $156.80  
  7 purchase totals = $638.16       $692.8050   $692.81   $692.80   $692.80