Running SELECT queries should be OK at any time.
It's those UPDATE, INSERT and DELETE queries that you worry about...
Against my own better judgement, I often execute UPDATE, INSERT and DELETE statements during the day. For example, when closing a billing cycle there are inevitably a few accounts that have the dreaded Last Closing Balance error. I run those queries right away from HQ Admin without even backing up.
I have inserted Alias records en masse during the day. (with backup)
I have made mass ILC changes during the day. (with backup)
Tom