Just how old are the washing machines you are all using? I have never seen any moderately modern washing machine (i.e. not older than 20 years), where it does not work like this:
You put your laundry into the machine and close the lid.
You put the detergent into a separate, smaller container near the top of the machine.
Only when you start the machine, the water is piped through the container for the detergent first, then to the laundry, where the detergent is already properly dissolved when it hits the laundry.
Obviously, this way it does not matter what you fill in when.
I think this is generally only the case for front-loading washers which are much more common in Europe (i.e. where you live). In America, top-loading washers are still pretty much the standard and you just add detergent directly into the tub at some point in the washing process.
I don’t know about WhyNot’s machines, but mine are something like 6 weeks old. And I’ve never seen a washing machine with a separate container for the detergent.
And I had the same thought as Aunt Flow*, followed up by Walloon’s thought…but then my third thought was, well the drains probably open up at the end of the wash cycle to let the water drain out. So would they re-close right after, or not until the machine was re-started?
Where’s the Maytag Man when you need him?
I let the water run, up to about an inch or so, then stop the water, then pour my detergent in all around, then put my clothes in, then start it again. For whatever that may be worth.
The drains at the bottom of the tub are closed until the rinse cycle, when they open to let the dirty-soapy water out. Then the drains reclose so that the tub can fill with rinse water. Then the drains re-open to drain the rinse water, and stay open as a rinse spray hits the clothes and the tub goes into hyper-spin.
If you use warm or hot water, it doesn’t matter much. The powder is slower to dissolve in cold water, though. Some machines start a cold wash with a few gallons of warm water to dissolve the detergent. That doesn’t help much if the washer is more than a few feet from the water heater; the first gallon or two from the hot line will be cold anyway.
I load my clothes, start the washer, then pour the liquid detergent into the stream of water that’s filling the tub from the top. But then, I don’t separate lights and darks, so I’m no Maven of Laundry.
I just went and checked the instructions on the underside of the lid (which I’d never read before). Step one is “Add measured detergent.”
It’s a Whirlpool Imperial Series Heavy Duty. (does anyone sell a light or medium duty anything?)
A little more thorough skimming would have revealed this sentence from the column:
Of course this was not a scientific study. I personally still use detergent out of habit. I put it on top of the clothes. I’ve never even read the side of the laundry box or the washing machine instructions to see which is the correct procedure.
My clothes always have a nice smell anyway because my body order contains pheromones that are a cornucopia for the discerning nose.
We have a septic tank, so we use the liquid. Start the water, pour in the liquid so it starts to get sudsy, then dump in the clothes.
Let’s clarify a few things about washers. The small holes being discussed are in the inner tub. There’s also an outer tub, which is fitted with the level sensing connection, auto dispenser connections (if so equipped), and the drain fitting.
In top loaders, the pump is sometimes bidirectional-during the wash cycle, it’s pumping through the little lint pickup screen/drawer (typical of older Whirlpool/Kenmore machines), yet on drain it’s turning the other direction, and the pump outer case allows the greater volume to flow up and out of the drain hose. As such, there’s no ‘darin to close’ per se.
Front loaders are often designed to gravity drain, and indeed have a drain valve which is solenoid operated (typical of Wascomat). The newest front loaders will have pumps for drain, as they will be used with laundry tubs and/or drain stacks, unlike the drain trough found at your local launderette.
I’m a fill it up with clothes, add water, and finally detergent person, as I don’t want caked up crap in the machine, or hot spots of detergent making my clothes look like a retarded tie-dye job. 
If that’s your worry, put the powdered detergent in first. The water flows into the tub from the bottom up. If the powder is at the bottom by the incoming water, it will have a better change to dissolve. If the detergent is at the top, it’s the last place the water reaches before the wash cycle begins, and it may stick together in clumps, or leave (harmless) powder marks on your laundry.