Like you said, they most probably instructed von Hammersmark on the requirements to some degree. But I don't think they gave that many instructions, apart from the obvious ones that it should be a quite desolate little bar in the outskirts of Paris, without a high chance of German soldiers hanging around. Of course they could have told her much more things, like "don't chose a basement bar, don't chose an attic bar, chose a bar where the bartender understands neither German nor English, chose a bar where the bartender doesn't have a rifle himself, chose a bar with the bartender belonging to the resistance, ...", but well.
But excluding those factors I'd say the tavern was a very good meeting point (ignoring the fact that it was coincidentally swarming with bad guys ;-)), especially since the basement-factor increased its desolateness and decreased the chance of someone from the outside seeing what conspiracies happen inside (and I for myself, being not a military strategist either though, would have thought it quite a good meeting place, too, hadn't Aldo told me it wasn't). And like said, she wasn't picking a battlefield, but a secret meeting place. In the end I'm not even sure there was really such high importance in that meeting place at all, given that they were just gathering and not doing a serious operation, which came only later on.
And apart from the above possible in-universe explanation, it was of course also needed in the story to drive the conflict between von Hammersmark and the Basterds due to her bad choice and to make the resulting chaos and standoff more exciting, without the remaining Basterds just shooting everyone easily through the windows. In the end they wouldn't have needed a public meeting place at all, necessitating the Basterds to dress and play as German officers. They could have just met in the woods or an abandoned house or wherever (or the house across the tavern, where all the other Basterds were), but that would have spared us one of the best scenes of the whole movie.
