You don't have to, the leaf springs have worked for 150 years. The reason I replace the trigger and bolt leaf with a wire spring has to do with the rate. A wire loads up a lot less at the compression end of its travel than a leaf of equivalent initial tension. With a Wolf wire spring the trigger will have a much more constant rate from take up through hammer drop and will have a lighter "pull". The bolt side of the leaf spring really loads up badly at the end of its stroke when cocking the hammer and when the bolt releases suddenly before the cylinder has quite indexed to the notch, it smacks the cylinder quite hard and I have found this to peen the bolt head into a mushroom which pretty soon becomes too fat to fit the cylinder notch. After carefully correcting three different Uberti cylinder bolts (one was my Dad's Cimmaron Bisley .45 Colt and was so bad I had to stone the sides before it would come out of the frame), I started replacing the leaves with wires and carefully tweaking them to give firm tension without the harsh slapping of the bolt or excessive trigger pull.
Another modification I like to make is to upgrade the cylinder base pin cross-bolt spring with a Wolf extra-power spring. Most of the Uberti SAA clones I've seen have two grooves around the cylinder base pin, one for normal operation and a second where the pin can be inserted deeper as a hammer block. Recoil causes the base pin to peen itself against the cross bolt, bump the bolt back against the weak spring so the sharp edge of the bolt's hourglass shoulder raises burrs all around the base pin's groove, and ultimately swage the base pin into the frame (seen that twice and numerous others that were beginning to chowder up before I intervened with stones, cold blue, and stronger cross bolt springs). One example required me to do some frame work and fit an oversized base pin after forcing the stuck one out with a hammer and punch. The replacement base pin was noticeably harder than the original which was akin to an iron nail. My Ruger New Vaquero came with a hard base pin, strong base pin cross-bolt spring, and wire trigger/bolt spring and has never had the slightest issue.
A pinhead-sized spot of blue threadlocker on each of the five grip frame screw threads is also a good idea as the darn things always seem to work loose, at least they do on mine.