The answer depends on:
1) Are you overclocking your CPU ? This creates additional heat which needs to be exhausted.
2) Are you over clocking your GPU's ? This creates additional heat which needs to be exhausted.
3) How many fans are in the case besides the "optional one" ? To get the heat out, choice between small number of fans spinning fast and noisy or large number of fans spinning slow and quiet.
If you building an enthusiast's box (25% + OC - 2 -4 GFX cards) with an enthusiast's Full Tower case (HAF-X, HAF-932, 1200 OR DF-85), I wouldn't add the optional door fan unless I was doing SLI / X-fire.
Put twin GFX, Hi OC into a mid tower (HAF 922 / Antec 900x) and that advice will hold but try and squeeze that into a smaller case such as the Illusion of 912 and you better add the optional fans.
A "bread and butter" retail PC for office work will survive with what it came with. Add gaming and add the fans.