Sorry for the third post in the day - this one shall be really short.
Thank you very much for your precious advice, Summerset and Shannon.
I followed Summerset's advice and washed a sample this afternoon, waited for it to dry, then ironed it according to Shannon's instructions. I don't know whether it's because I'd serged the swatch beforehand (it was fraying and I didn't want it to fray into nonexistence in the washing machine!), and maybe that pulled the fibers, but the swatch ended up significantly coarser. The warp threads became much more apparent.
It's difficult to photograph the difference so here are a couple of pictures at different angles. The washed swatch was laid onto the unwashed fabric, along its grain.
Can you see what I mean? Do you think the serged edges caused that?
In any case, I certainly don't want the whole fabric to look that way.
Anyway, I still have time to decide... All I did with the dress today was read through the pattern instructions and figure out which alterations I wanted, and how to do them. Then I left to co-sing at a French-Portuguese mass, which was very moving and uplifting.
Hope you're all having a lovely weekend.