I'm getting ready to swap out my bars, and figured it would be a good time to replace my brake cables while I'm at it. Since I'm doing that, I figured I might as well replace the derailleur cables as well. I'm planning on getting some jagwire cables for the brakes, but am not sure about the derailleur cable. My derailleur cables don't have any housing, it's bare cable the whole way. Since this is the case does it really matter what kind of cable I get? It seems as though what separates some cables as better has to do with the housing and whether they're sealed or unsealed.

I had good luck with Clarks cables. They're cheap but good enough for most bikes. If you want to look at other brands, try to find the ones that feel smoothest when you run them over your fingers. They feel more like a hose and you do not feel the strands on them. Manufacturers of these better cables run the cables through a machine that roll presses and smoothes them down.

Downtube shifters?

You'd still want to replace the little loop that goes into the RD. I would just go down to the LBS, walk past the shelf display and head back to the mechanics' area. Explain what you want to do and they'll give you two derailleur cables and a short piece of housing from the big generic box (probably made by Jagwire). Don't forget to ask for some end crimps.

There's always housing if the levers are on the handle bar. Downtube levers don't need any housing until the 8-12" loop from the chainstay to the RD. You still want a decent die drawn wire, preferably stainless, because there can be a decent amount of friction in that little loop. Since you need so little housing, buy 2 inner wires, and ask the shop to cut you a small length off a bbulk roll of housing.

What kind of bike is this? I can't think of any derailleur bikes without shift cable housings.
On a derailleur bike the most critical housing is the short housing at the rear derailleur.

You guys were right, there is a short section of housing at the rear derailleur. I never noticed it before, as it's only about 6 inches or so.

a cable Die drawn after being finished , flattens the outside wire surfaces. so will offer
less drag, thru the housing.

I do like them soldered before cutting , Stainlesscable that is tough to do,
though with a Low temp [400 degree] Silver content solder Its possible..

Zn treated plain steel is easier to solder ,a wipe of grease will keep it from rusting

soldering makes re installing the cable easy, so re applying grease later is easy.

That lower loop of housings tends to pick up an inordinate amount of road gunk and should be replaced. Even though it is a short run of housing, don't forget to grind or file the ends flat and install the proper ferrules. Make certain that there are not any housing wire strands sticking out. Poor preparation is a frequent cause of difficult-to-diagnose shifting problems. Prepare the brake cables with the same care for best performance.

It needs to be about 13 inches.