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 $234.60   12.376%   $263.6341   $263.63   $263.65   $263.60  
  Purchase #2 $205.64   8.509%   $223.1379   $223.14   $223.15   $223.10  
  Purchase #3 $67.65   9.391%   $74.003   $74.00   $74.00   $74.00  
  Purchase #4 $166.88   12.856%   $188.3341   $188.33   $188.35   $188.30  
  Purchase #5 $55.60   10.424%   $61.3957   $61.40   $61.40   $61.40  
  Purchase #6 $124.74   12.344%   $140.1379   $140.14   $140.15   $140.10  
  Purchase #7 $30.95   10.874%   $34.3155   $34.32   $34.30   $34.30  
  7 purchase totals = $886.06       $984.9582   $984.96   $985.00   $984.80