I don't believe there should be an automatic success on a 20 or failure on a 1, because this negates several class abilities, especially for contested rolls. I had an 11th level rogue in my game who's minimum roll for stealth was 23 (all rolls less than 10 became a 10), so I'd check the monster's maximum Perception. If the roll couldn't get to 23, I wouldn't even bother to make any rolls. If it could, I'd check to see if they even got to the minimum, and only then would I bother the player to roll. It actually saved time, and the player felt vindicated that his specialization (which came at the cost of other options) was useful.
Additionally, this takes away from the rule of the DM determining the outcome of an action and only using dice when the outcome is uncertain. If a 20 is always a success, or 1 a failure, then there is ALWAYS uncertainty, allowing players to do some really silly things (like jumping to the moon, or tripping over their own feet while walking) 5% of the time.
Additionally, this takes away from the rule of the DM determining the outcome of an action and only using dice when the outcome is uncertain. If a 20 is always a success, or 1 a failure, then there is ALWAYS uncertainty, allowing players to do some really silly things (like jumping to the moon, or tripping over their own feet while walking) 5% of the time.