It doesn't make any difference imo. I used to have a flatmate who'd swear that if you put the milk in first then you burn it when adding the water
but I don't think that's true. And I've never found the dissolving thing to be an issue. But I normally put the milk in last anyway because I usually judge how much I want by what colour it goes, but I'm not too fussy about that so I just do whatever most of the time.