While I understand that the carpet installers have no desire to own this problem, I suspect that they are underpromising in the hopes of covering their behinds or the hopes ofoverdelivering. If the trends and the risers are secured to the stringers with screws, nothing can move relative to anything else, and therefore the squeaks should virtually disappear, in much the same way as a floor is screwed into the joists to prevent it from squeaking. Repairs are normally from below (with wedges)when there is a desire to have no visible screws from above (exposed oak stairs, for example). It sounds like you are prepared to have the screws, as they will be concealed by the carpet. Because wood shrinks and swells with the seasons, there is always the potential for some squeaks, as there is with screwed floors. Therefore, when you remove the existing carpet, and before yourescrew the treads, sprinkle talcum powder onto the stairs, and tryto work it into the joints. Should there be any relative movement between components,the talc will act as a dry lubricant.