If your build only uses one Skill to do all your damage, you put 20 Hero Points into the Skill and two +10 Skill on the Build to get the +40 Skill Points.
If you find an Eternal Item that has a +20 Skill you need, and all of the other affixes work with your Build, you can go that way.
If your Build will use 2 or more Skills, or a lot of Proc affixes, you could use the two +5 All Skills (affects all weapon skills used, including Procs) and any +10 Skills as needed.
there are also many builds that only have their Weapon Skills at +20 to save space for other affixes.
here’s a post showing the few Crystals that can be used on Eternal Items.