Room requests are made note of in your reservation. Every effort will be made to accomodate your request, if possible. During peak times, it would be more likely that specific room requests could not be met than it would be at lesser busy times.
Your room assignment is usually made the night before you arrive, so that everything is in order by the time you check in. After all, they don't want you to spend a lot of time in the lobby, but would rather have you out enjoying yourself and spending money.