Well don't do stepped sets just to do stepped sets. That is, everything should be fairly loose until you are in there doing it.
Now lets imagine a situation.
You got 235 on the bar and you have a base volume of 3x5.
So you get in there and you do:
235 x 5 x 1 set
You could have added reps to that but while you were doing it you felt like it was just TOO light, right? So you stop at 5, and add weight.
So you do 240 x 5 x 1 and that is demanding and fairly close to failure. Take a rest…
240 x 6 x 1 (this time you new you had 5 more in you but you surprised yourself with a 6th rep).
That would be a fairly typical scenario and just the kind of thing you'd expect to happen. Since you started with 3x5 but then added weight and added one rep, technically that is double progression. There is no rule that says you have to do it with every set. In fact there are no rules as long as you progress.
In fact, as soon as you impose rules it stops being what I've described and starts being a "program" right?
Heck, you could add a set of two if you felt you had that. Still legit.
For the next workout you might start with 240 and go from there. So you can always add weight…at any time. And you can always add reps…at any time. And you can always add sets….at any time. It's really a reactive way of training. Best not to have any more than a loose plan of attack.
On the other hand, with a three rep set I wouldn't drop the reps down on the last set, or the last two sets, just to add weight. Better to keep with at least the base volume and add to that while possible adding weight.
Because when you dip the volume…what is likely to happen is you just slow down progression. Depending on how much the volume drops, the added weight may not represent that much progression. That is why the idea of having a base volume is important. Look at it this way…it you can do 235 for 15 reps, is it really such a big deal to do 240 for say, 10 reps? What happens next workout? You dip the volume again? Or you hit 240 for 15 maybe?
Let's imagine two scenarios, each representing two workouts. Two workouts don't mean much, of course, but for our purpose of description it's better to keep it simple:
Workout One: (weight x reps x sets)
235 x 5 x 1 (not too hard, not too easy)
235 x 7 x 1 (got easier….)
235 x 7 x 1
Added reps to two sets…single progression
235 x 8 x 1
240 x 5 x 1
240 x 5 x 1
Second Scenario (your scenario of adding weight to last sets but dropping volume)
235 x 5 x 1
240 x 3 x 1
245 x 2 x 1
First of all..it doesn't seem like a big deal but from the first scenario the volume has dropped by almost 50%! Half. Now for a very advanced trainee…and I mean very advanced…adding 5 and then 10 pounds to the bar can be a huge deal in itself. But a very advanced trainee wouldn't have been ABLE to do it and he wouldn't really be training this way with primary movements that much.
Workout Two A:
240 x 5 x 1 (oops…shit hard)
235 x 4 x 1
235 x 6 x 1
Not really as good as the first scenario. Why? Dropping the volume back helps the most after you have built up more volume and built up a bit more fatigue. Best case scenario is you've even added weight and volume. Then once you drop back to base volume, even with a little added weight, you are, in effect, getting a break.
But if your volume is low to begin with, dropping it lower just to add weight may not be an advantage. It's possible of course, for it to be just the right thing to do. Say you haven't been getting your sleep or eating quite right. That little break on volume may be just the thing, even with added weight, to spur you on to new heights. But it's not predictable and it makes basic progression too difficult to allow such fluctuation in volume and intensity. I.E. it starts to resemble weekly undulating periodization and for a beginner that is just silly. Well I think it's silly for most anybody, lol, but that's just my humble opinion.
Or if you did in a very straght-forward fashion it would start to look like short linear periodization. Again…not my cup of tea.
So to sum it up…try to maintain at least the base volume. Dipping a little below that is perfectly ok sometimes but do not make a habit of dropping volume to add weight. Not with SDT that is.