Flexible hose to rough in valve in tub

I have a quick question. I got a roman tub faucet pretty inexpensively and it has the rough in valve with solder connections.

I have PEX supply lines and a drop ear elbow thing so I can connect it solidly.

Can/should I solder on threaded connections and then just use the flexible supply line to bring water from the pex to the rough in valve or do I need to solder it?

Valve is kind of like that one.
http://www.**************.com/Prod/Delta-R2707-Roman-Tub-Whirlpool-Faucet-Rough-In/23990/Cat/1089

Flexible lines would be way easier as I would not need to line every thing up perfectly, but I don't know how reliable they are.

supply

The are not designed for installation is concealed areas, especially since they are often sealed up forever. And they are also too small internally to give a good flow for a roman tub application.

Many, but not all, valves that have threaded male inputs will also allow a pipe to be soldered into it directly. Try a piece of pipe and see if it will fit inside. If it does, you can clean it and the pipe, flux, then solder in a stub, then connect to the pex with whichever transition fitting you want.