(x) is a piston
[x] is a redstone block
--] is a piston extension

I built the wall of redstone from the bottom up (after I had placed the pistons so that everything below the top row of pistons was in a BUD state.

(a)[a]
(b)[b]
(c)[c]
(d)[d]

I expected the pistons themselves to produce block updates as they extended or retracted. For example, updating (d) would have done this:

(a)[a]
(b)[b]
(c)[c]
(d)--][d]

(a)[a]
(b)[b]
(c)--][c]
(d)--][d]

(a)[a]
(b)--][b]
(c)--][c]
(d)[d]

(a)[a]
(b)--][b]
(c)[c]
(d)[d]

(a)[a]
(b)--][b]
(c)[c]
(d)--][d]

However, only (d) extended.

Starting with the initial wall and updating (c) and (b), in that order, you end up with:

(a)[a]
(b)--][b]
(c)--][c]
(d)[d]

Once (b) is extended, (c) is no longer powered. We know that pistons do not update when extending, otherwise (c) would have already retracted. If (c) is updated, it will retract [c] which will power (d), so it is expected that (d) will extend when we update (c):

(a)[a]
(b)--][b]
(c)[c]
(d)--][d]

However, (d) did not extend.

To determine that pistons only produce updates when they retract, you can place two pistons next to each other, force one into a BUD state, and then toggle the other one. You can also a block for the piston you are toggling to push and pull to verify that it does not prevent block updates. The logical conclusion is that the update is produced before the redstone block that is being pulled is in place.