This is what I do (well, what I have Picard do). I keep disc #, but all tracks are numbered sequentially
What I meant is that I would expect that on the box of the box-set the first track of the Live cd will be printed as #1, not as #7.
After all, it is the first song from the live set.
I assumed you meant that you will use sequential numbering all over the box-set. That would result in the first track of the Live set being #7.
But then that would differ from what's on the cd label, and the print on the box-set.
(I am probably misunderstanding you though)
Another example of how I probably do things a bit differently:
The main reason for the numbering being continuous here is the limitation of cd technology.
I would use the numbering: Jump for Joy 1-12, The Man I Love 1-12, Bonus tracks: 1-3
And then use sub-set numbering 1-3 for them.