For me personally, it depends.
How many other people are in the air, what are we doing, how current am I etc.
After a sequential jump I have had no problem with accepting a center dock at, say, 2K or 1500 if we're doing 2-way.
I've also been known to be part of a "post-stack" which is complete at the altitude previously indicated, but flown down to 100 ft or so. I do that only with people I really know and trust.
Offset docks, that ends at 3-3.5K if we're doing 2-way and isn't happening at all after a larger sequential jump.
I don't coach people for their first canopy formation jumps (I'll leave that to @IJskonijn) , but I'd like to submit that the minimum docking altitude would also greatly depend on what gear everyone is on. For the purpose of this discussion I regard bumping endcells as "docking" where newbies are involved.
The most important thing to know is, cross your legs once you reach the minimum altitude you're comfortable with and people know to stay away.