Some do and some don't. Some trust the Watchtower completely and some don't. Most have nagging doubts about some doctrines even if it's on an unconscious level. However when a Jehovah's Witness has doubts, these cannot be expressed to anyone. Disagreeing with Watchtower doctrines even privately with friends is not allowed. The punishment for disobedience is severe. Unhappiness, depression, fear, guilt, and other negative emotions can lead to mental illness.
Most Jehovah's Witnesses blissfully believe they are free and exercise their conscience in all matters. They don't believe that the organization controls them.
However based on the information contained in the secret handbook for elders only, it is evident that the organization must be obeyed when it comes to holiday celebrations. If the Watchtower changed their position and told everyone they must celebrate the holidays, Jehovah's Witnesses would be forced to obey or suffer the consequences of being disfellowshipped and shunned
So the real question is not what Jehovah's Witnesses believe, it's what the Governing Body of Jehovah's Witnesses says is 'the truth' at this point in time. Jehovah's Witnesses are not allowed to lean upon their own understanding or question the authority of 'God's visible organization' (the Watchtower). 
Rebellion against the organization is thought to be rebellion against God. That's why faithful Jehovah's Witnesses never disagree with anything the organization says. It's not allowed. Everyone must speak in agreement and be 'like-minded'. Of course we know that's not normal behavior in any free society. See Mind Control