One should NEVER, EVER use lookups at the table level. They are a crutch for people who don't know how to create a query or write code. Once you get to the point of doing these two things, the evils of table level lookups will come to haunt you. Combo boxes on forms solve the problem of data entry. The user gets to see a meaningful value but behind the scenes, the numeric foreign key is stored and that is what you need for joins in your queries.
FYI, although there is no prohibition against a RowSource query containing a join, it would be extremely rare for this query to contain anything other than a single table.