First thing in the morning is not good, because it requires people get in in time, and so it becomes a gate on starting work if you are on time until the latest person arrives. Also kind of useless to do until the morning email triage is completed.
Midday doesn't work, I've found, because it is impossible to carve out a consistent time to meet. External sources always try to clutter up 10-2 with calls and meetings that involve herding cats and aligning schedules over which there is little control. Also tends to blow into lunch time and people get hangry and cranky.
End of the day at least serves as a terminus to the day, and since standups are mostly useless, skipping it to sneak out and play golf or whatever tends to be a common outcome.
