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 $80.98   11.305%   $90.1348   $90.13   $90.15   $90.10  
  Purchase #2 $106.83   10.153%   $117.6764   $117.68   $117.70   $117.70  
  Purchase #3 $119.70   5.685%   $126.5049   $126.50   $126.50   $126.50  
  Purchase #4 $126.78   13.714%   $144.1666   $144.17   $144.15   $144.20  
  Purchase #5 $109.44   11.873%   $122.4338   $122.43   $122.45   $122.40  
  Purchase #6 $85.33   5.520%   $90.0402   $90.04   $90.05   $90.00  
  Purchase #7 $115.54   12.214%   $129.6521   $129.65   $129.65   $129.70  
  7 purchase totals = $744.60       $820.6088   $820.60   $820.65   $820.60