By the higher levels there's only a handful of cantrips/orisons that are still useful anyway, so that rule probably won't make much of a difference for a 17th-level wizard.
You should probably include a minimum amount, as the rule would otherwise unduly penalize casters starting with 15s or lower in their casting stat (which would mainly affect low-level casters, who are most in need of cantrips/orisons).
Or allow a bonus cantrip/orison for each bonus 1st level spell they get for having a high stat. So a wizard with a 20 int would mean a total of 6 cantrips a day. However, having a low stat means you get no penalties.
We were considering it slightly different. We never actually got around to making the house rule, but were considering something along the lines of:
For every 5 caster class levels, a character can, with 15 minutes preparation, switch one of the prepared orisons for a different orison that he/she knows.
With my group, I simply worked out the progressions to expand the chart detailing bonus spells for high casting stats to include 0th level spells. While this grants every caster a lot more cantrips/orisons, it really helped to underscore the high-magic setting we're going for.
Link: Bonus Spell Progression
With my group, I simply worked out the progressions to expand the chart detailing bonus spells for high casting stats to include 0th level spells. While this grants every caster a lot more cantrips/orisons, it really helped to underscore the high-magic setting we're going for.
Link: Bonus Spell Progression
This makes a lot of sense to me.
I wouldn't take anything away from caster with low scores, just give a small bonus to those with high scores.