Payment in advance is the standard where I live (southern Alberta). Mainly because I don't charge any type of deposit, and if someone leaves without notice, then I am assured not to be left without my paycheque. Most people pay me at the beginning of the month; my contract states that payment is to be received BEFORE the first of the month. There are a couple parents I need to "remind" on the first, but most of them have been great.
The only exception is daily drop-ins, which usually pay me at the end of the day.