I think the most important thing about being AST is being adaptable and having the situational awareness to understand where a card should go. AST is the only job in the game where you need to have some knowledge of how every single class/job in the game plays so you can make a well informed decision with your card selection.

By straitjacketing yourself to a specific set of rules for your card usage, you're just undermining your maximum potential as an AST because you don't consider specific aspects of fight. Certain cards will be more beneficial in specific fights and even in specific parts of a fight. Spire and Ewer lose their effectiveness at the beginning of a fight but can be invaluable later in a training dummy intensive fight like T8 or if someone dies and is Raised, Spire / Ewer can get them back in the fight much faster.

Don't give yourself rules to use card, give yourself options and see how they mesh with the fight you're in.