Endgame Guided Builds
Your Build Is Bad & I Should Feel Bad
(If you're not following one of the recommended Guided Builds, you can skip this section.)
In the interest of simplicity, the Guided Builds included in this guide stayed away from a few things that, if I were making an optimized build, I would never have skipped. At this point you've played through the game, accrued some currency, and have a basic handle on the game's mechanics, which means it's time to revisit your build and make some tweaks.
The Family Jewels
The Guided Builds skip Jewel Sockets except where necessary for Threshold Jewels that modify your skills directly. The reason for this is that you might not find, or want to shop for, jewels on your own during the main storyline, but at this point you've surely found a few or can buy them if not.
As a general rule, any Jewel Socket that is either directly along your path, or only one point off of it, is a no-brainer. It is very easy to get a Jewel that is as powerful as two points on the passive tree - even just spamming Orbs of Alteration will get you that much! Jewel Sockets that are two points off your main path are still usually worth it, but for those you'll need somewhat-more-expensive rare-rarity jewels (typically at the cost of a few Chaos Orbs apiece).
You can either devote your next few points to picking up these skipped sockets, or use some of your Passive Respec Points to recover enough points to do so. Look for Jewels with Increased Life along with relevant damage modifiers (like % Increased Minion Damage if you're playing a Minion build or % Increased Critical Strike Chance if you're playing a crit-centric build).
Tipping The Scales
Depending on the quality of your equipment, different modifiers and support gems will have different power levels. If, for example, your weapon is very bad, Added Damage modifiers are much better than they will be once your weapon is very good.
The Guided Builds are built around the assumption of downright terrible gear. As a result, some of them use support gems that would not be optimal for better-equipped versions of that build. Once you've gained a few items, consider firing up Path of Building and seeing if some alternatives might be better than the support gems suggested as part of the main guide.
In general, expect Added Damage bonuses to become weaker, and crit-related bonuses to become stronger, relative to Increased Damage the better your gear is.
It's Party Time
Many players play PoE solo - but maybe you don't! The Guided Builds assume solo play so that you won't be dependent on outside help, but that doesn't mean you can't play with a friend.
If you play with friends, consider adapting your build to party play. For example, you may be able to swap out a Curse for one that is slightly weaker for you but much stronger for your party members - perhaps you use Cold Projectiles and your party member uses Fire Projectiles, so you switch from Frostbite (which reduces Cold Resistance) to Projectile Weakness (which increases Projectile Damage).