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 $16.60   7.304%   $17.8125   $17.81   $17.80   $17.80  
  Purchase #2 $79.28   9.520%   $86.8275   $86.83   $86.85   $86.80  
  Purchase #3 $37.89   6.587%   $40.3858   $40.39   $40.40   $40.40  
  Purchase #4 $10.86   12.654%   $12.2342   $12.23   $12.25   $12.20  
  Purchase #5 $148.60   7.348%   $159.5191   $159.52   $159.50   $159.50  
  Purchase #6 $11.98   8.290%   $12.9731   $12.97   $12.95   $13.00  
  Purchase #7 $239.83   5.406%   $252.7952   $252.80   $252.80   $252.80  
  7 purchase totals = $545.04       $582.5474   $582.55   $582.55   $582.50