Hi Ian,
In my opinion there is no best time of day to cast.
We are only really able to accurately measure gross ROM (ie not in degree-increments, although this still goes on

in both physio and podiatric circles, in spite of robust evidence that ROM does change over the course of one day due to circadian variation), so no best time of day for this either.
I don't believe measuring or casting at a specific time of day would have any bearing whatsoever on outcomes. It certainly would not influence my prescription protocols.
Cheers,
David