Neutral IV Tharja

Go ahead and build her. She can fight well at Neutral, and you can merge her into a +spd one later. (+atk is also acceptable in the absence of +Spd, as long as she's not -Spd.)

I'd say build her now. Neutral Tharja is better than a -Atk or -Spd Tharja, and you could always merge, like Kratos said.

If you want to run a red tome, then it’s not a bad thing to build her, especially since she’s not bad at a neutral IV. But her tome could come in handy for other red tone users you will find down the road (Leo, Halloween Nowi when she comes back, Katarina, etc.)