JOEL - It is important for us to pray about things we hear from the church leaders and allow the Holy Ghost to witness to us that what they are saying is true. We need to have faith that God will not allow His prophet to lead us astray on matters that are important to our eternal life. I don't know of anything of eternal importance that any prophet in the past has said that has turned out to be wrong twenty years later. Past prophets have expressed their opinion on other things that may or may not be correct, but they have been about things that are not really essential for our salvation and exaltation. And usually when they are expressing their opinions on something they are careful about telling us that it is their opinion.
Knowing the kind of person President Hinckley is and what he has experienced through his life, I am willing to follow his council on any religious matter, even if it is just his opinion. We have been promised that God will bless us for it. But at the same time, rather than blindly follow, I will ponder his words; compare them to what the scriptures say; and receive confirmation from the Holy Ghost through prayer.
For more important matters, we can know that the Prophet's words are the truth by confirmation from the other General Authorities of the church. Matters of eternal importance are always discussed among the First Presidency and Apostles before the Prophet makes his final decision by way of prayer and revelation, before he says anything to the general membership of the church. Joseph Smith once said:
“The Lord will not reveal anything to Joseph that he will not reveal to the Twelve or to the least and last member of the Church as soon as he is able to bear it.” (See Teachings of the Prophet Joseph Smith, p. 149.)
We also need to remember that what our current prophet says is more important to us than what past prophets have said. God continues to give us knowledge and understanding about things when He believes we are ready to receive it.
A scripture in The Book of Mormon says,
"For behold, thus saith the Lord God: I will give unto the children of men line upon line, precept upon precept, here a little and there a little; and blessed are those who hearken unto my precepts, and lend an ear unto my counsel, for they shall learn wisdom; for unto him that receiveth I will give more;" (2 Nephi 28:30)
As we continue to "hearken unto His precepts", God will continue to reveal more of His mysteries to us through His prophets, so we can better understand Him and His teachings.
So what a past prophet understood 100 years ago may not be exactly the same as what our current prophet understands now. But even though we may increase in our understanding of things, the basic doctrines and ordinances of the Gospel of Christ will not change.