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 $124.83   11.782%   $139.5375   $139.54   $139.55   $139.50  
  Purchase #2 $61.47   3.253%   $63.4696   $63.47   $63.45   $63.50  
  Purchase #3 $157.76   3.278%   $162.9314   $162.93   $162.95   $162.90  
  Purchase #4 $28.61   6.774%   $30.548   $30.55   $30.55   $30.50  
  Purchase #5 $52.36   6.214%   $55.6137   $55.61   $55.60   $55.60  
  Purchase #6 $168.45   6.681%   $179.7041   $179.70   $179.70   $179.70  
  Purchase #7 $157.79   9.286%   $172.4424   $172.44   $172.45   $172.40  
  7 purchase totals = $751.27       $804.2467   $804.24   $804.25   $804.10