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 $240.58   10.845%   $266.6709   $266.67   $266.65   $266.70  
  Purchase #2 $115.75   9.372%   $126.5981   $126.60   $126.60   $126.60  
  Purchase #3 $30.74   7.456%   $33.032   $33.03   $33.05   $33.00  
  Purchase #4 $110.65   13.322%   $125.3908   $125.39   $125.40   $125.40  
  Purchase #5 $195.40   8.542%   $212.0911   $212.09   $212.10   $212.10  
  Purchase #6 $186.66   12.108%   $209.2608   $209.26   $209.25   $209.30  
  Purchase #7 $5.93   11.392%   $6.6055   $6.61   $6.60   $6.60  
  7 purchase totals = $885.71       $979.6492   $979.65   $979.65   $979.70