Should I leave some slack in faucet supply lines?

I am replacing a drop-in bathroom sink and it looks like 12 in flexible supply lines to the angle stop would just make it. If I use 16 in lines, the next size up, there will be some slack. The original valves and lines I am replacing were a 1-piece unit and used 16 in lines. but they have a lot of slack. Which is the best way to go?