Depends if you have triple IRS at your FMS' disposal, or like the old 73-3 or 73-4's, you have only 2. The first instance gives the FMS the electronic equivalent of the "cocked hat", averageing out drift errors, while the latter only allows a single IRU input to the FMS at any one time.
If your in a part of the world that provides you with little opportunity for rho/rho updates, and, like most early glass flight decks, you don't have GPS imput, a full align seemd sensible, even on a twenty minute turn around. I do 'em all the time.
That help????????