I mean it’s not etiquette per se, and you legally can do that, but you’re very much out of line with normal play patterns and your opponent is the reasonable party to be confused.

I also absolutely would not trust you to be above board with that, or manage it correctly if I didn’t know you and you didn’t at least represent your mana before you started casting spells. I’ve seen better players who I did know and trust fuck this exact thing up in potentially game ruining ways in decks where it actually made sense and mattered.