IMO (and by all means follow what your instructors recommend instead), is to do one commercial nav, then your hour building, then the rest of the navs required for CPL.
By doing at least one nav first, you'll have an idea of the standard to work towards on your hourbuilding, while once you get back you'll be able to nut the remaining navs out quickly.
I did 1 CPL nav the day before I left for a 6 day trip, 1 nav the day after I got back and the final CPL navs the week later.