Well, ideally, your strength is increasing as your weight on your primary movement increases. So you (again, ideally) will be working at roughly the same percentage of your maximum work capacity.
Now, this won't be true in the short term with a lot of the programs out there that work in waves, but unless you program your assistance to follow the same wave, it will still hold true in the long(er) term.
That was just a bunch of shit I spewed out off the top of my head, but I believe it to be true.
But it's just assistance, so it's nothing to lose sleep over. I just think that overall, you want to be sure you're adding resistance (or volume) to your assistance lifts, even if it's not every every session.
And as anecdotal evidence, I've found that when I keep my assistance numbers increasing, it tends to be when I make my best gains overall.