I've had a fitbit for a couple years, and started with the default and changed my stride settings a few times before I used the method I used. Right now my fitbit distance is consistently accurate or very close for either walking and running distance compared to a measured track, treadmill, GPS, etc. The first method I used was counting steps while walking a short known distance, and it would result in too high of distance estimates for me (though I am sure it works fine for some people).
Where someone wears the fitbit can make a difference in how many steps it picks up. The distance can still be pretty good the fitbit is worn as you prefer and you use the fitbit counted steps in calculating your average stride length. Some people wear it on their waistband, some on a bra strap or cup, some clipped to a shirt pocket, some in a pants pocket and all of those will cause some variation in how it counts the "steps". That is really the main reason I suggest actually wearing the fitbit while you calculate stride length. It is probably fine to set your stride length before your fitbit arrives, but if you find it is giving distances too high or low compared to the actual distance, that is something to consider.
I was not saying that having a fitbit effects one's stride length. Just that if your goal is to set your "fitbit account" stride length to make the fitbit stats as accurate as possible, it may be more effective to do that while you are wearing the fitbit in the way you will typically wear it and using fitbit's step count over the distance.