I had the same doubt with my twins, who just went through the application process: is it appropriate to take them out of school for open days?
I asked their headteacher, who said that it was appropriate, and indeed advisable for them to see the school in "operating" conditions - i.e., during the school day rather than on weekends/open evenings.
What we did in the end was to visit during the school day the ones that were a realistic option for us, and leave for weekends/evenings the ones that would make it to our list of 6 but were either less realistic due to distance or seemed less desirable.
Anyway, worth having a chat with your headteacher as they may have different views.
Good luck with your choice! it's daunting, but I came to the conclusion that there are so many good schools in the area that it's unlikely to get it really wrong...