You read that it was usually a best practice to check code in daily or more frequently to the main branch. Is this always true?
No, it is not always true. It is advisable to check code in regularly, but you cannot always check code in this frequently.
In some situations a developer would be terminated for checking in code daily. Ideally code should be checked in daily or more frequently than this, but we know every job is different. Journalists and researchers make recommendations, but they are not your employer. It is considered a "fundamental practice" to check code in often (according to page 399 of Continuous Delivery).