Don't carelessly waste regalia on relic weapons you aren't sure, or very close to sure, that you'll use. Herc axe of course is nice enough to save up for.
But for Mordred? If you're thinking 'oh, I'll get a relic weapon for Mordred just cause I have her, even though I have no plan for what to do with it', then stop right there. Change your thought process right away. On the other hand, if you're looking at Mordred's relic weapons, want what they specifically do, and intend to get actual use out of whichever one you pick, then fair enough, go for it.

If nothing else, regalia can always be used to limit break Herc's axe to crank up its burst effect, buy a different element version of Herc's axe, or for Final Limit Breaking stuff. 10 regalia is one of the components needed to FLB a valid* union event or advent SSR weapon. Of course, how much of a big deal 10 regalia is varies from person to person.

*short version: weapons gain FLB eligibility over time starting from the introduction of Guild Orders and orichalchum in August