Should never be an issue unless the tyres on the rear are unsafe or your driving like a tool.
Here's a video highlighting how it can quickly become an issue when the tyres on the rear aren't unsafe, and no one's driving like a tool.
EDIT: It seems you can't embed videos to start at a timestamp - go to 1:38 to see the demonstration.
It's perfectly possible to have all 4 tyres on the car deemed to be "safe", ie, good condition, good quality and with decent tread depth (they state "half worn", so that's about 4mm...a tyre still well within it's serviceable life).
But
comparatively, if the rears have less tread depth, are the most likely to be the first to lose their ability to shift any given amount of water on the pavement, and the result is the car quickly becomes unstable.
Watch until you see the cars driving side by side, at the same speed, on the same corner. The only difference is tyre placement, yet the one with the worn bias toward the rear quickly becomes squirrelly, and then loses control, when driving at moderate speeds on a gentle corner. The advice is purely about trying to balance the probability so that it's always your front tyres which give up first, so you can feel it and do something about it.