The answer is relevance to decision.
Managerial accounting reports are designed to aid internal management in making informed decisions. Therefore, the primary guideline for these reports is their relevance to the decision-making process. This means that the information provided should be pertinent and useful for managers to plan, control, and make strategic decisions.
While aggregation of data can be a feature of managerial accounting reports, it is not the primary guideline. Aggregation refers to the summarization of data, which can be useful but is secondary to the relevance of the information for decision-making purposes.
Managerial accounting reports can indeed be special-purpose, tailored to specific needs of the management. However, this is more of a characteristic rather than a guiding principle. The key guideline remains the relevance of the information to the decisions that need to be made.
Timeliness is an important aspect of managerial accounting, as timely information can significantly impact decision-making. However, it is not the primary guideline. The most crucial factor is that the information must be relevant to the decisions at hand, even if it is timely.