You’re correct, that’s the idea. You should only have to pull the UCA and adjust the heims if you can’t get the alignment right using the eccentrics, ie, you didn’t have the heims close enough to the alignment specs you wanted to finish with the eccentrics.
The first time you install them it might mean pulling the UCA’s an extra time to reset them, it depends on how extreme an alignment you’re going for and how straight your car actually is. They may not be the same side to side, and the more caster you want the less equal the heim lengths will be. Some caster is already built in too, so there’s some margin for error. If you do your own alignments that’s no big deal, if you take your car somewhere it might mean an extra trip because a lot of places won’t touch aftermarket parts they didn’t install.
The only exception to that I’m aware of is the E-body Hotchkis UCA’s. They relocate the front UCA mount for better geometry, but that means you don’t use the front eccentrics. Which means all the adjusting is done with the heim on the front mount. On A bodies I’m not aware of any that do it that way though.