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 $19.66   6.695%   $20.9762   $20.98   $21.00   $21.00  
  Purchase #2 $131.77   9.874%   $144.781   $144.78   $144.80   $144.80  
  Purchase #3 $16.74   3.789%   $17.3743   $17.37   $17.35   $17.40  
  Purchase #4 $250.51   9.314%   $273.8425   $273.84   $273.85   $273.80  
  Purchase #5 $95.80   5.274%   $100.8525   $100.85   $100.85   $100.90  
  Purchase #6 $58.67   8.639%   $63.7385   $63.74   $63.75   $63.70  
  Purchase #7 $58.25   11.832%   $65.1421   $65.14   $65.15   $65.10  
  7 purchase totals = $631.40       $686.7071   $686.70   $686.75   $686.70