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 $15.70   11.389%   $17.4881   $17.49   $17.50   $17.50  
  Purchase #2 $70.99   12.825%   $80.0945   $80.09   $80.10   $80.10  
  Purchase #3 $210.90   9.675%   $231.3046   $231.30   $231.30   $231.30  
  Purchase #4 $39.54   11.113%   $43.9341   $43.93   $43.95   $43.90  
  Purchase #5 $186.94   13.505%   $212.1862   $212.19   $212.20   $212.20  
  Purchase #6 $18.89   10.204%   $20.8175   $20.82   $20.80   $20.80  
  Purchase #7 $97.80   13.627%   $111.1272   $111.13   $111.15   $111.10  
  7 purchase totals = $640.76       $716.9522   $716.95   $717.00   $716.90