I think you're right sazlocks that in the end things will be the way they'll be... I suppose the thing for me is to be informed enough and have a plan enough, that should things be going as you hoped, or only just off plan, then you have a chance to have a say, whereas if you decide not to think about it at all, you really don't stand a chance of influencing how things go.
I didn't have a birth plan (other than that I wanted a home birth, which didn't happen, though that was fine). And I wish I'd thought more about having a managed third stage beforehand and really understood the options. I didn't, but it was a decision made in the moment, and nearly resulted in me having the placenta removed surgically having given birth without so much as gas and air. I didn't realise that there was a time limit on a natural third stage, and if you missed the time limit you didn't get the injection, or even the drip, just straight to theatre with you... I managed it just within the time limit.
In fact, it was knowing that intervention was about to come, both with the 2nd and 3rd stages, and knowing I really didn't want it, that gave me the extra motivation to do it myself.