The gospel is not the dead letter of the law but the live spirit. But in order to try the spirit we have to have standards. It's clear that each disciple did paraphrase some in quoting Jesus. But I doubt if they actually made any important substantive changes. I think they would have enough respect for the truth to avoid that. In some cases I think different gospel writers just picked up on different points as Jesus spoke. It would be hard to mentally note everything said, and they didn't write the gospels until well after the events. So, for instance Mathew remembered that Jesus mentioned the abomination of desolation, and Luke remembered that He mentioned the armies around Jerusalem. Both were true and together we get a broader picture of what Jesus said and meant.