It's really what you can manage - my FS will do closer monitoring if you want but I don't have the time around work so I'm happy with my CD21 test confirming I O'd. But a CD21 test isn't the rule... you can just adjust based on cycle length and get tested later in the cycle once you know when you are O'ing.
I would make an appointment as soon as you can get one at a time that works for you. Maybe aim for the end of your cycle - and of course cancel if you are successful. I once rang to see about cancelling my appt as I wasn't sure about timing and the receptionist basically said to me - the waiting list is long enough that I could call them that morning and cancel and they would easily get someone to fill my spot (without charging me for cancelling either).