If casting a circle before your reading makes you feel more secure, puts you in a ritual mood, or whatever reason you want – do it! There’s nothing wrong with casting a circle before doing a reading, especially a larger reading
I agree with Megan! If you feel pulled to cast a circle, then go right ahead. There are no rules to divination. I carry a circle if I’m doing a reading with my deities (death God & Goddess and I don’t want any spirits from the underworld to tagalong.