Now I might be wrong here, but as far as I know - there is no FG gacha that a guaranteed 6* ticket.
For DMM points yes, but not for FGems. So if its flower gems we are talking about, and if the gacha gives a guaranteed rainbow coin.... eeeeeh...maybe ? I mean if you are pilling them up slowly and you dont have any gacha you are saving for specifically.. id say go for it.