The answer to this has been different at different times in US history. For the most part, the reservation system has worked to encourage native cultures, but there have been times when the opposite was true.
In general, reservations allow native cultures to survive more than they might without reservations. By putting Native Americans together in one area, the system allowed them to live together and keep their culture. Had they moved to cities and assimilated, their cultures would have been lost to a greater degree.
However, there have been times when reservations have been used to try to destroy native culture. This was especially true in the days of reservation boarding schools where the idea was to "kill the Indian to save the man." In those times, US policy was to use the reservations as an efficient way to eradicate native culture.