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 $39.63   11.178%   $44.0598   $44.06   $44.05   $44.10  
  Purchase #2 $160.93   13.626%   $182.8583   $182.86   $182.85   $182.90  
  Purchase #3 $76.70   6.446%   $81.6441   $81.64   $81.65   $81.60  
  Purchase #4 $72.36   6.296%   $76.9158   $76.92   $76.90   $76.90  
  Purchase #5 $165.91   9.648%   $181.917   $181.92   $181.90   $181.90  
  Purchase #6 $134.61   10.154%   $148.2783   $148.28   $148.30   $148.30  
  Purchase #7 $242.41   6.163%   $257.3497   $257.35   $257.35   $257.30  
  7 purchase totals = $892.55       $973.0230   $973.03   $973.00   $973.00