Depends on what you're doing. Most of the time, when you call up a uke, you're doing so because you want to teach or discuss a technique; so when he comes up, you want him to attack in a particular way. By that Imean; if you're teaching kaitenage tenkan (I love that one!), it's no good for uke to attack with a haymaker; you want a same-side wrist grab.
There are also plenty of times you'd want uke to choose the attack, and plenty of times you'd want her to tell you what she was going to do as well, such as teaching ma-ai, or when demonstrating how many different ways you can use ikkyo. It all depends on what you're doing.
BTW - there's no better compliment to a teacher than saying 'so-and-so made me think about something..." Thanks.