* Building Flourish isn't very useful anywhere.
* Wild Flourish can be useful in two situations:
1) You're fighting a mid-tier monster and do not have the FMs to self-skillchain with Reverse, but the monster isn't going to live long enough for you to get the FMs.
2) You're fighting a monster that you can one-shot with Wild -> WS but WS -> Reverse -> WS would be major overkill
I find I mostly use it in case 2 situations.
Inside Abyssea, you will want to use the Razed Ruins atma and spam Evisceration
Outside Abyssea, you will want to make an AGI WS set and use Exenterator
If you get the chance, unlock Pyrrhic Kleos so you can make self-darkness with Reverse and Evisceration (assuming you haven't yet.)
Realistically, Rudra's Storm isn't a very good unstacked WS, so Twashtar isn't really *that* good of a dagger even at level 90. You might be better dual wielding STR/Atk magians than going Twastar (90)/STR Dagger.
Here is how dancer daggers shake out at the moment-
Aluh Jambiya - Great buyable weapon for the main hand (or offhand). This is approximately equivalent to a STR dagger and is cheaper/much faster to make.
STR/Atk Magian - Great weapon. 11 STR and 22 Attack
Oynos Knife - Great offhand for when you don't have a mage Hasting you.
Pugiunculus - I doubt you have to worry about this one because it's a rare drop off a Legion monster, but it is probably the only offhand competitive with STR/Atk magians
AGI/Eva Magian - I have 99 Mythic / 99 Twashtar, and this dagger is still part of my collection just in case I'll be using a hybrid evasion set. It's such a huge amount of Evasion that it's the most efficient way to exchange DPS for Evasion.
Twashtar (99~90) - Good main-hand daggers. Offer a unique WS opportunity for DNC (aka a reason to use the Flourishes III category at all), but Rudra's isn't that impressive otherwise, but might be worth using for the double damage aftermath
Twashtar (80-85) - Really just not very good, but fortunately the 90 upgrade trial is pretty easy.
Coruscanti - It's about as good as a level 90 Twashtar and maybe better as an offhand. I do not recommend doing Qilin for this. ( http://www.ffxidb.com/voidwatch/178 )
Terpsichore (99) - Combinations main-handing Terpsichore typically come out with about 25% more damage than other combinations assuming you don't waste too much AM3 time.
Gusterion - Very interesting knife. I don't really know what to make of it.
If you have no pocket WHM, I'd main-hand Aluh Jambiya and offhand Oynos Knife.