You will love having your machine flush with the desk top - It's much easier to sew almost anything that way, unless you need the free arm, and even that is fairly workable if you have the removable insert that gives access. If you have a machine that has the bobbin cover below the level of the machine bed, as I do on my 40-yr-old Bernina, then you need easy access to that area. If it's tightly fitted into the desk top, it would have to be lifted out each time you change a bobbin.
Another advantage to a rolling chair is that you can place a little table or TV tray of some sort at an L to the main table, and you can easily turn to do ironing, trimming or pinning. If your floor has carpet or a delicate floor, you'll need to get a chair mat to roll on. It's probably a good idea even without the rolling chair.