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 $236.61   9.822%   $259.8498   $259.85   $259.85   $259.80  
  Purchase #2 $61.38   12.640%   $69.1384   $69.14   $69.15   $69.10  
  Purchase #3 $175.98   5.462%   $185.592   $185.59   $185.60   $185.60  
  Purchase #4 $184.60   13.532%   $209.5801   $209.58   $209.60   $209.60  
  Purchase #5 $186.48   11.682%   $208.2646   $208.26   $208.25   $208.30  
  Purchase #6 $5.69   11.556%   $6.3475   $6.35   $6.35   $6.30  
  Purchase #7 $109.26   10.796%   $121.0557   $121.06   $121.05   $121.10  
  7 purchase totals = $960.00       $1,059.8281   $1,059.83   $1,059.85   $1,059.80