I go with the idea of is this a dramatic roll? I.e Would the chance of failing it have an impact in normal game play. If it would then I would say roll for the chance of complications. If not then I would handwave it as you are so good it is just an everyday task for you.
In the example of a Mentat trying to recall some information on spice production in preparation for a negotiation. I would say that the character need not roll if they want the basic info. If they want more they need to risk it.
If they were trying this during the negotiation with the NPC staring at them across the table then I might want a roll as a complication could be damaging.
Also players may want to roll as there is a good chance of gaining Momentum from it.
With almost all the automatic successes they replace a rolled dice with a dice set to 1.
So for a standard understand test to recall something the character with Mentat Discipline would roll 1d20 and set their other d20 to 1. So they start with 2 successes plus whatever else they roll.