[dredmorbius]

The key with deads (or any other lift) is to get the form right first.

Doing full ROM lifts will improve your flexibility. I find this holds true more for squats than deads, specifically (you're getting pressed down with the bar, and have to retain proper back/spine alignment), but it's all useful.

If your form's breaking down, you're lifting too heavy.

What causes other than muscle weakness do you identify as causing posture issues?

[vidarh]

If you're not flexible enough, you're not able to do full ROM lifts with proper form.

Once you're flexible enough to actually do the lifts properly, I agree you will see it improving further. But if you're flexible to do the lifts properly, you don't really have a problem.

When I started was not physically able at all to get reasonable depth in my squats before I would either need to stop or curve my back, no matter the weight. I also could hardly bend over while keeping my back firm - it'd be like stretching a violin string along the back of my legs.

A couple of months of stretches from my pysiotherapist got my squats from about 60kg to about 140kg in less than 6 months, after I'd spent the previous 6 months stalled completely because any attempt to go higher led to total form breakdown, and a lot of time before that with abysmally slow progress.

[dredmorbius]

Doing squats (and, if necessary, additional mobility work) will increase your flexibility.

Doing light / BW / goblet squats is very low risk, and will help iron out your form.

My point is that if your form is crap, and you're using any added load, then you're going too heavy. If your form is crap at BW (and adding a modest load, 45-95# for most trainees) doesn't iron that out (sometimes a bit of "bar discipline" does help sort out form), then, yes, work on form.

My experience is that you want to eliminate the obvious and simple stuff first (Occam's Razor), and that for most trainees I've seen, starting with simple cues and light weights addresses form issues sufficiently. KISS.