It really depends on the machine.
When I was using a Whirlpool belt drive top loader, I'd add the clothes, fill the tub with water, and then add the detergent. That way I could add more, or less, to get a thin layer of suds.
When I used a GE filter flo, I'd add the clothes, put the detergent in the filter tray, and then start the washer. The tray would act as a dispenser and help dissolve the detergent before it hit the clothes.
When I got the Neptune front loader pair, I'd add the detergent to the dispenser and let the machine do the rest.
When I got a Miele front loader, I started out adding the powdered detergent to the dispenser. Then I started to notice that the detergent (Sears HE) wasn't dissolving in the dispenser, leaving a long block of hard semi crystallized detergent in the chute and complicating rinsing etc. After that, I started adding the detergent to the drum, then the clohes, and then start the washer. The powder probably wasn't dissolving in the dispenser because the machine is hooked up to cold water only.
All these methods seem to work well.