How do I know when an accounting number needs to be exact for CPA exam purposes?

Start with the user and the decision. A number needs a higher level of precision when an error could affect external reporting, tax compliance, covenant compliance, compensation, audit conclusions, or a trend that users rely on.

For low-risk internal work, a documented estimate can be enough if a more exact number would not change the decision. For external reporting and audit work, the standard is not perfection, but it is also not casual rounding. The work should be precise enough that the statements are not materially misstated.