Please do go and visit the grammar schools you are thinking of applying to before you send your DC to sit the exams. It will save you a lot of hassle when trying to decide in what order you should list the schools on your CAF later. There are quite a few parents struggling with their decisions simply because, by their own admission, they have not seen the schools.
I did my first round of grammar school open days in year 4, on my own, just to see what they were like and which of them were worth showing to my DS (plus I didn't really feel like dragging him with me at the time). I liked what I saw, so in year 5 I took him to all of them. Visiting each school twice gave me a chance to ask all those questions I didn't think about during the initial visit and confirm (or otherwise) my first impressions about the schools.
Sutton Grammar's open day is usually in early September, Wilson's and Wallington boys in June/July. Nonsuch (girls) and Wallington girls also have open days in summer.
I am not a big fan of open days, but for me, they were worth the time spent on them and definitely a big factor in prioritising the schools.