I have a theory that when you plateau/stall or even just feel like crap (also beat up around the joints) in the gym for a few weeks, it's your body forcing a deload on you. It does this until its recovered then you get a couple of weeks of good training in before it happens again.
I first thought about this when I trained with a highland games/strongman competitor who "didn't believe in deloading". He pretty much believed in insanity every single week. I deloaded and was mocked for it (and various other things). So, just observing his patterns of progression and regression, it seemed to make sense.... "if YOU don't deload yourself, your body will deload you!" - i like to think there's some sense in that statement. Granted, the guy DL'd 290KG, squatted 260KG, benched 175KG - I believe he now deadlifts 320KG (he moved away). Anyway, he would progress for 2 weeks tops, then would get weaker (and angrier) over the course of about 3 weeks, then some injuries would creep up, force him to accept lighter weights for a week or 2, then he would make progress again. I trained with him for almost a year and this is pretty much how the year went for him. Actually, this guy defied a lot of what you see on paper - such as doing heavy (often 1RM attempts) on squat, DL and bench EVERY week (I also seen him do a pendlay row with 190KG!).
Anyway, in some cases you're just not eating or sleeping enough. I think the less experienced you are, the more likely it is that you just aren't eating/sleeping enough.
I just feel you can take control of your volume/fatigue, and schedule it in, using one of the various methods of doing this.
So, to answer the original question.... It depends... Also, a lot of programs actually finish on a deload so, it just depends on the program, what you were doing before it, how you feel, etc.
If you are starting a new program and it has some new exercises in it, I think it's worth while taking a deload week before you start it, and just groove the movements you will be training. I think it mainly goes by feel - you want to feel fresh when you hit week 1 of a new program.