Whether a partner is chosen during the cortina or the start of the first piece in the tanda depends on whether people are there just to dance, or they want to dance to that music. Forewarning the next tanda, as illustrated in the
DJ thread, if available, changes that.
It also depends on the context of the milonga. It can just be for dancing and to choose available dance partners, or to socialise with some dancing. Sometimes those conversations can be more important.
It's one reason why cabeceo/mirada can be good if the sole purpose is to dance.
The key aspect is to be available and clearly seeking a dance at the salient points if you wish to dance.
All of which is complicated by the fact that – even in today's world of social media and text messages – you never know
for sure who's going to be there. I don't travel much for dance, but I have done it, and I've been surprised to run into a friend from another city in a third city where neither of us lives. I've gone to my local milongas which have long-established crowds only to see
nobody I know because it's summer, they're all away on vacation, and the attendees are tourists on
their vacations. Even when you see people you know, someone might have had a bad day, or be playing host to visitors, or be on some sort of date they'd prefer to have uninterrupted. Newly met people could end up being more interesting in conversation than on the dance floor. And as the night wears on, you might wear out and be more interested in talk than dance, or warm up and grow more interested in dance than talk.
In brief, you could show up intending to be social and end up just dancing, or show up intending to be a near-anonymous dancer and end up being social.
I think that a lone dancer – leader
or follower – needs to be more flexible than usual on all of these points. It's fine to say, "Since I know nobody here, I'm going just to dance," but you might in fact meet someone you know who has a reputation you
don't know, and not be aware of the message you send by socializing with them. You might make friends the moment you walk in the door – ditto. You may show up at the hour most dancers there see as "too early" or "too late." You could miss invitations by not being social enough with strangers . . . or by being
too social with them.
My experience is that, overall, being social is the best bet initially, but once you start dancing as a consequence of that, it's wise to focus on the dancing more than the socializing.