My preference is that during the arranging of the appointment we cover fees and method of payment. I have paid a deposit on occasion (usually $100 or less), to reserve time in advance when it is not same day or much later.
If the payment is cash, sometimes I will leave it out near the bed or take it out upon arrival and set it on the nightstand next to the bed. There are certainly times when I got to the appointment, and we got into it really quick, and I paid after and I like those times because it is more intense.
If the payment is electronic, I prefer to do it after because I want the option to tip. I usually take cash as a backup in case I have a problem sending payment.
Sometimes, I will politely ask if they prefer before or after when I arrive. Most say after.