canklecat

I'm a cheapskate and often reuse cable housings through two or three cable swaps. But I usually replace the small loop of cable housing around the rear derailleur with every new cable. That's the one that wears quickest, due to the tight loop and proximity to grit and debris from the rear tire.

The other cable housings don't have any sharp bends or tight loops so they seem to last longer, as long as there's no infiltration of grit, or the cables themselves didn't oxidize or rust. I had galvanized cables on one hybrid and those things were like sandpaper when they were old and badly wore the insides of the cable housings. So swapping in new cables and housings were a big improvement in shifting and braking feel.

But I've reused the same set of mid-range Jagwire and Shimano housings through two or three cable swaps on a couple of road bikes (other than using new cable housing for the rear derailleur loops). So far, so good.

Another thing that can improve the feel of shifting, especially with the front derailleur, is cable liner or similar tough, hollow plastic tubing to protect the cables from those metal guides on the bottom brackets. Doesn't matter whether it's old school brazed on guides, or those stamped metal guides riveted to my old Trek carbon fiber frame -- cable liner helped smooth out shifting. Recently I couldn't find my loop of clear plastic cable liner, so I used the hollow plastic stems from cheap cotton swabs. Perfect. Yeah, they're bright red and look funny. But they're very durable and do the same thing. I snip 'em to fit the guides, and use an ice pick to open the ends to ease inserting the cables.

rydabent

If the housing on you particular bike or trike doesnt have narrow radius bends you may not have to.
Example old RDs usually had a 180 degree turn where the cable attached. New RDs like mine have a pretty much straight in lead to the RD that probably doesnt wear the housing much at all.