In general, we observe the no school, no gym policy, as well as 24 hours fever/vomiting free.
Granted, there was one time recently where DD was not feeling great on a Monday - she went to school, but I kept her home from practice that night. The next day she still seemed wiped, so I let her stay home from school. By the end of the day, she was feeling so normal that she was trying to tumble in the house (and was driving me insane). I let her go to practice that night. That said, it was more of a light "cold" than anything (no fever/vomiting, etc).
Sometimes, it's tricky. Once she complained of a tummy ache after school. She'd been having some anxiety, and it was "that one practice day of the week" with "that one coach" who was much more.... strict..... than all the others. She seemed otherwise fine (perky, etc), so I assumed she was just anxious. Wrong. She was vomiting within 4 hours of getting home from practice. I felt awful!!