It's not so much to keep the arm there till you're ready so much as to remind them that you're not defenceless as you step. If you do it properly then it's their fault if they get hit.
IMO you should always step YOUR stance length - forward or back - if not then of course your never going to end up back where you started (as a side issue obviously) but if you start training differnet stance lengths for differnet directions then have a think about how this might affect other areas - such as Kata for example - or are you planning to have a whole range of stepping distances that you've tailored to suit your partner ?
The Hiki-Te arm is supposed to shoot out as you initiate the step back anyway - if it's not then in fact you're not blocking properly
Also my point about the opponent not really training properly is that they (IMO) should not be relying on "tricks" to overwhelm you but should be working on stepping and punching fast enough with no "telling action" that they catch you with a clean technique from a "ready" position - just like the first attack.
Just my thoughts/opinions obviously - perhaps it's a "Seki Tradition" to Blitz attack - at least until you "wise up"

Will be interesting to hear what the other Seki Instructors think about this.
I think that you'll be surprised also at how much it actually sharpens up the blocking action too.
Jim