Yes, but, no!....
I'll try to explain.
Taking it all in in the Atlas Mountains
You should absolutely take Altitude Mountain Sickness (AMS) seriously, at the end of the day it could (and does) lead to death. The best way to avoid AMS is to:
Have a sensible itinerary - don't try and ascend too much in one day
'Climb High, Sleep Low' - Where possible climb higher than you're going to sleep that night. Sometimes, but not always possible.
Trek nice and slowly, especially up the hills. There's no need to over-exert yourself.
Stay hydrated. You're looking for around 4 Litres a day. Soups and tea can count toward this fluid intake.
Be as fit as you can be going into the trek - Fitness isn't scientifically proven to help you acclimatise, but the fitter you are, the less a given day will take out of you, therefore leaving more in the tank for your body to put towards adapting to altitude.
Eat well - being fuelled will help you recover between the days.
Go on expedition with good Iron levels as a deficiency has been attributed to poor acclimatisation.
With all this said, there is still always the possibility your body won't acclimatise, regardless of the fact as to whether you've been to altitude before. As we say, 'Altitude doesn't discriminate', and just sometimes your body may not get the physiological changes done in the timeframe you have.
And so, if you do everything in those bullet points to your best ability, and you could still get AMS, is there any real use in wasting energy worrying about it? I believe not. As an expedition leader it's a tough thing to convey, but I see people worrying too much about AMS even when they're doing everything we ask of them. Many hyper-focus on any possible symptom that may align with AMS - such as a headache. Yes, this is present when you have AMS, but it also occurs when you're tired, dehydrated, have had too much sun or not slept well. My concern is that people spend so much time in their minds on the expedition worrying about stuff like AMS they actually miss some of the highlights of the trip they're on. Walking at altitude isn't easy, you will feel short of breath at times, your heart rate will be higher for the relative effort at sea level, you may even have the odd dizzy spell, understanding all this before you go will certainly help you feel more at ease when you're there.
The Portable Altitude Chamber packed and good to go
My best advice would be:
Get as fit as you can be for the trip
Get into a routine of completing all of the above bullet points as best as you can
If you're on a commercial expedition trust that their itinerary has been designed appropriately, and if you're unsure, seek clarity before you go.
Be transparent with your expedition manager. Don't just lock things away in fear that if you have AMS that means it's curtains for your trip. Firstly, talking to someone about how you may be feeling will make you feel better. Secondly, it helps paint a picture for your expedition manager and they will be able to act or assure you accordingly.
Here at LMG Treks & Expeditions we take a proactive approach against AMS. From daily Oxygen saturation monitoring through to building in extra days into our itineraries where some expedition companies don't (such as our Mount Toubkal Trip). On all our expeditions over 4,500m we take our own Portable Altitude Chamber (PAC). A PAC is essentially an inflated tube that a person suffering with severe AMS can get into and it provides a pressurised environment that helps alleviate symptoms. We of course hope to never use this very expensive air bed, but it's an insurance policy worth having should we need it as the impact it can have can be quick and very effective.
So, if you're on on a trek to altitude soon, do what you can in your control, and make sure you enjoy yourself! The chances are you will be fine, so don't worry.
If only we could have all been born at 3,500m like the Sherpas at Namche Bazaar!