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 $20.83   13.273%   $23.5948   $23.59   $23.60   $23.60  
  Purchase #2 $168.70   3.309%   $174.2823   $174.28   $174.30   $174.30  
  Purchase #3 $59.64   9.868%   $65.5253   $65.53   $65.55   $65.50  
  Purchase #4 $190.72   11.439%   $212.5365   $212.54   $212.55   $212.50  
  Purchase #5 $74.36   4.675%   $77.8363   $77.84   $77.85   $77.80  
  Purchase #6 $234.35   9.235%   $255.9922   $255.99   $256.00   $256.00  
  Purchase #7 $168.29   8.697%   $182.9262   $182.93   $182.95   $182.90  
  7 purchase totals = $916.89       $992.6936   $992.70   $992.80   $992.60