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 $13.53   12.107%   $15.1681   $15.17   $15.15   $15.20  
  Purchase #2 $42.86   13.236%   $48.5329   $48.53   $48.55   $48.50  
  Purchase #3 $247.80   7.598%   $266.6278   $266.63   $266.65   $266.60  
  Purchase #4 $81.76   6.498%   $87.0728   $87.07   $87.05   $87.10  
  Purchase #5 $144.92   9.498%   $158.6845   $158.68   $158.70   $158.70  
  Purchase #6 $33.68   7.583%   $36.234   $36.23   $36.25   $36.20  
  Purchase #7 $7.41   5.241%   $7.7984   $7.80   $7.80   $7.80  
  7 purchase totals = $571.96       $620.1185   $620.11   $620.15   $620.10