Should you have that determined before the first day of practice?

It depends on your situation, the size of school, how levels are organized, and so on.

For example, when I was coaching at a 3A high school, all juniors and seniors were on the varsity. However, the exceptional freshman and sophomores would get moved up to varsity. Sometimes this happened before the season started and other times it happened in the middle of the season.

Also, the kids that didn't get much playing time on varsity would play in JV games. There were only about 10 JV games so they didn't get as much playing time. Also, the games weren't quite as serious in regards to preparation and so on.

Maybe if you can share the details of your situation we can give your our opinion?