For my personal preferance, i love to do mine in a circle. I do a circle casting weekly, and once a month. I usually find that meditation, and grounding helps me before hand.
Although anytime with or without a circle, you can do divintation. Divintation doesn't require a circle, but it may help the intuiton,and protectiveness. Some i know, cast whenever, and wherever. Other take a ritual bath, few take it in a circle. The whole thing is up to you!