Do I need to fill the drum when loading washing machine?

It's definitely better to wash full loads whenever possible. It saves time, energy, water, detergent, wear and tear on the washing machine, and the load is likely to spin more smoothly too. Also, many modern washing machines won't even spin at maximum revs unless the load is nice and evenly balanced. The problem is we don't want to overload it either, and a drum crammed with laundry just won't wash properly if there's no space for the washing to move about. (One of the symptoms of overloading can be white streaks of undissolved detergent in folds of washing and of course poor wash results).

The useful guideline (mentioned here - How do I avoid out of balanced loads in my washing machine? ) is to leave a gap of roughly a hands width between the top of the laundry and the top of the drum but depending on how you do this, and the type and mix of laundry, you can end up with different results. However, different types of laundry have different maximum load sizes and only cottons should use the maximum capacity of the drum as explained here - Laundry comes out of washing machine badly creased?

One variation for example (apart from the variation on people's hand sizes) would be dependent on whether or not you pat the laundry down (and how firmly) before measuring the gap. Some laundry also shrinks down much more than others when wet, so when water is added, a load that looked full can suddenly appear only three quarters full. There's no way to get the loading perfect, and it's partially a case of trial and error and experimentation, but it's a good idea to be aware of the benefits of washing fuller loads and the hand guide is a rough and ready estimator.

The object is to fill the drum but not cram it so much that the clothes have no room to move around when the drum revolves. Also, bear in mind that the drum capacities quoted by manufacturers are most probably for a perfect load, maybe all the same fabric and similar in shape. In the real world the exact weight of an optimum load will vary depending on the shape, size, and mix of fabrics.

Conversely, if you don't pout enough items of laundry in most modern washing machines will either refuse to spin or only spin at a reduced rate - Washing machine won’t spin just one item or very small loads