I guess there isn't going to be any agreement on this one. So, for what it's worth, here's what I do and what I teach.
Generally, I teach students to turn base, and then slow down and lower flap as soon as the base turn is complete. For many students, I leave it there.
But for the more competent students, I go one stage further once they've got the hang of this, and teach them to take the wind into account. So if there is a cross-wind from the dead side, this will hold you back on base leg, so you can wait a little longer before selecting flap. On the other hand, if the cross-wind is from the live side it will create a tail-wind on base leg and speed you up, so you need to get everything set up a little bit sooner. This may well mean selecting flaps during the base turn. I have no problem with this - it's what I do myself, and if my students make an informed decision to do the same that's fine by me.
I have no doubt that my opinion will change the first time I encounter asymmetric flaps, though!
FFF
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