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 $22.66   8.617%   $24.6126   $24.61   $24.60   $24.60  
  Purchase #2 $91.55   5.674%   $96.7445   $96.74   $96.75   $96.70  
  Purchase #3 $6.81   13.636%   $7.7386   $7.74   $7.75   $7.70  
  Purchase #4 $27.76   8.728%   $30.1829   $30.18   $30.20   $30.20  
  Purchase #5 $216.45   13.611%   $245.911   $245.91   $245.90   $245.90  
  Purchase #6 $119.65   7.411%   $128.5173   $128.52   $128.50   $128.50  
  Purchase #7 $15.68   7.735%   $16.8928   $16.89   $16.90   $16.90  
  7 purchase totals = $500.56       $550.5997   $550.59   $550.60   $550.50