autovacuum to prevent wraparound question

Does the auto vacuum process scan the whole database every time it runs (when triggered to prevent wraparound), or is it able to skip pages for which every row has already been frozen?

We have a large database that consists primarily of two large tables that receive a lot of insert records, but no rows are ever deleted or changed. Can we expect the autovacuum to always take longer and longer as those tables grow, scanning every row every time it is triggered to prevent wraparound, or is the vacuumer optimized to skip the parts that have already been frozen?

Thanks,
Natalie