For patients who have received vaccinations post-chemotherapy based on their vaccine titers, is it necessary to repeat those titers over time to make sure the titers haven't waned?

Unless the patient is absolutely free from any infections- even “colds”, sore throats, and sinusitis- I would consider it prudent to check, especially for pneumococcal polysaccharides and RSV. Re-immunize if titers are not protective. They should get annual flu and COVID immunizations.