I have found that the majority of my clients love to be able to self schedule! But, I admit that it was scary in the beginning to let go of that amount of control.
A couple of things I have discovered:
As a new therapist using self-scheuling, don't start allowing self-scheduling until you have all of your 'regulars' entered onto the calendar and know you have them set up on your calendar for repeating appointment slots. Otherwise.....other clients will self-schedule into that slot.
I block off some 'prime time' spots in order to save them for clients that I know need them - i.e. my 5:30 and 6:30appointment times aren't available just for 'anyone' to take - but I will release/unblock them for the next week on this Friday evening for anyone to schedule into if I haven't used them for scheduling my usual and customary clients.
I encourage clients to set regular repeating appointments at their visit with me.
I let my clients know that my appointment openings are 'first come/first serve' if they are not set up on regular repeating appointments. Also, if they cancel an appointment for next week - they can choose another time that I already have open - and to check back often if I don't have anything available. Since my schedule is very dynamic, it can change on a dime - and they can take advantage of that fact.
I only open up for brand new clients to self-schedule if my schedule is pretty light - i.e. I need new clients! If I'm pretty full, I don't allow new clients to self-register, and instead they are instructed to call my office.
If anyone takes 'advantage' of the scheduling system i.e. consistently cancels at the last minute - I remove their username and password and take the privilege away from them - and inform them that they will need to call to make their next appointment - that they can't schedule online.
I think you will find that the majority of clients respect the ability to self-schedule. If they do not - use that as a diagnostic tool: They likely have boundary/narcissism issues that need to be addressed :)