The choice is purely subjective and depends on the objective of the lesson, the age, level, and impulsiveness of each student. In general, it is observed that when your students know that you have a full picture of who sees the solutions before or during the execution of an assignment, they do not make inappropriate use. The relevant teacher information includes; a) accurate recording of the date and time the student views the solution in their stats and b) recording of the date and time when the student has viewed the solutions in the context of self-assessment.
In any case, if the purchased textbook you are teaching contains extra material for educators, you may send new exercises, unknown to the students, that are only communicated to them in the context of assignments. You can also hide the solutions in each lesson, at one or all levels.
Note that if you are using the notebooks you created for the course, you can choose to partially hide their contents in the same settings tab by clicking Manage Visibility of notebook contents. Students will not be able to see, for example, the contents and exercises of the upcoming week.