> Where do you draw the line?

Embezzling money.

And no, this is not an opening for some roundabout logic implying slacking off is the same as embezzling wages... you can't come up for a non-harmful explanation for embezzling money: the word literally means "steal or misappropriate".

You can effortlessly do the same for someone slacking, as the comment you replied to did.

> Most managers would expect junior team members to speak out. If it's being addressed, the manager will likely tell you that he or she is aware and its being addressed.

Most managers would expect you to share in terms of how it affects you, not some vague air of "they don't do anything":

So not: "I feel like they always have excuses for doing nothing"

Instead: "I needed X, Y and Z on separate instances from this coworker and was unable to get that"

(Or specific to excuses: "I expected X, didn't get X and they gave excuse Y")

It a good practice to do that anyways since it protects you from potential alienation and/or embarrassment if it turns out your idea of "them doing nothing" is dead wrong.